Examination of exfoliation mechanism intunnel lining joints
Tsuyoshi Tsujimoto1 and Sadao Kimura2
1Graduate School of Kanazawa Institute of Technology
E-mail : t.tsujimoto@venus.kanazawa-it.ac.jp
2
Depertment of Civil and Environmental Engineering, Kanazawa Institute of Technology
E-mail : s.kimura@neptune.kanazawa-it.ac.jp

Deformation of a tunnel lining is caused by external and internal factors, which may also result in thepartial exfoliation of concrete lining. To reduce this type of exfoliation risk, it is necessary to shed light on
the mechanism of exfoliation phenomenon, which, however, has yet to be done because of the use ofplain concrete for concrete lining. In this study, the authors focus on the eluviation of calcium hydroxide
in concrete, collected concrete cores from a sound part and a damaged part of a tunnel lining that had
served for 40 years, and measured the amount of calcium hydroxide and calcium carbonate contained in
the cores collected. As a result, the damaged part with water leakage was found to be experiencing theeluviation of calcium hydroxide. Based on the experiment results obtained, the authors propose the exfoliation process of concrete pieces activated by eluviation of calcium hydroxide in concrete lining.

1. Introduction

Photo.1 Exfoliation of concrete on inner lining

Eluviation of calcium

Letent stage

Crack and void development

The longer the service life of a tunnel, the more often its concrete lining suffers partial exfoliation. For
example, there have been several reports on the incidents of partial exfoliation from construction
joints in concrete lining1)．Because the exfoliationof concrete pieces is an operational risk, when itoccurs, the repair costs, compensation for an accident resulting in injury or death or any other expenses incurred as a result could mean a huge loss
to the tunnel administrator. To reduce the risk ofconcrete exfoliation, it is necessary to shed light on
the exfoliation mechanism and develop a prediction
technique.
In this study, the authors formed a hypothesis about
the concrete exfoliation mechanism. Figure 1 outlines the hypothesis, which holds that a concrete
lining starts experiencing concrete exfoliation as it
undergoes the following continuing and progressive
phases:Phase 1: Calcium hydroxide is eluviated2)3)4) bywater infiltrating from ground through concrete
lining cracks created by certain unknown causes.
Phase 2: The eluviation of calcium hydroxide
makes the vicinity of cracks porous, which further
accelerates cracking.
Based on this hypothesis, the authors focused on
the eluviation of calcium hydroxide in the concrete
lining of road tunnels. The amount of calcium hydroxide and calcium carbonate in a sound part

Calcium hydroxide eluviation
stage

Calcium hydroxide eluviation
acceleration stage

Enlargement of eluviation ofcalcium hydroxide
Initiation of calcium
hydroxide eluviation

Outbreak of penetrating crack
and initiation of water lekage

Hardened cement paste
becomes porous

In-service period

Crack acceleration

Progress of autogenous shrinkage

Formation of closing crack

Environmental actions as contributing factors
External forces as contributing factors

Exfoliation

Fig.1 Eluviation of calcium hydroxide andcrack development

(i.e. without any damage such as cracking) and adamaged part (i.e. with penetration cracks causing
water leakage) were measured and compared to
study the difference, which clearly showed theeluviation phenomenon of calcium hydroxide.

Clinker compound
3CaO・SiO2（alite）
2CaO・SiO2(blite)

Water＋H2OHydrate
nCaO・SiO2・mH2O（Calcium silicate hydrate）[n≒1.2〜2.0]⇒Ca(OH)2（Calcium hydroxide）

2. Calcium hydroxide eluviation theory anddeterioration of concrete lining

Fig.2 Cement components 7)

2.1 Calcium hydroxide eluviation theory andprogress of concrete lining deterioration
In this study, the “eluviation of hydrates” means aphenomenon where hydrates in concrete dissolve
into soft water, causing the concrete formation tobecome coarser."5) The calcium hydrate in cementpaste is largely consisted of calcium hydroxide andcalcium silicate hydrate,6) as outlined in Fig.2.7)
According to Minagawa, et al,8) calcium hydroxide
undergoes calcium eluviation before calcium silicate hydrate does, as outlined in Fig.3. When calcium hydroxide disappears from cement paste, calcium silicate hydrate dissolves as shown in Eq.1,
and new calcium hydroxide is generated inside thevoids. When calcium hydroxide dissolves from calcium silicate hydrate and the calcium concentrationin cement paste is reduced, calcium silicate hydrate
undergoes rapid decomposition and forms Silicagel.
This chemical reaction theory is applied to thecase of concrete lining. Based on a number of past
exfoliation case examples and exfoliation observation data, an assumption was made that damage to a
concrete lining would be accelerated in a mannershown in Fig.4. Because construction joints in atunnel lining involve more complicated execution
processes than other parts of the lining, they areprone to inherent quality variations and defects,
such as cavities and micro cracks. Draining channels, which discharge groundwater from ground, areoften provided in construction joints, which often
reduces the lining thickness locally. In many cases,
it is found that these minor defects eventually develop into penetration cracks and cause water inflow. This objective evidence indicates thatxCaO・SiO2・(x+0.8) H2O→(x-y)CaO・SiO2・(x-y+0.8) H2O+yCa (OH)2 (Eq.1)
Hydrates

Water

nCaO・SiO2 ・mH2 O
Ca(OH)2＋H2O⇒Ca(OH)2，H2O

3CaO・Al2O3・3CaSO4・
32H2 O3CaO・Al2 O3・CaSO4・12H2O

Fig.3 Dissolution of hydrates
Drainage channel
Ground side
Concrete lining

Inner side

Concrete lining

Presence of poor in quality
Construction joint

Deterioration accelerated by waterinflow

Crack vicinity weakened and becoming porous due tocalcium hydroxide eluviation

construction joints are greatly affected by theeluviation of calcium hydroxide caused bygroundwater inflow.

Outbreak of exfoliation

Fig.4 Exfoliation process of lining concrete

3. Verification test on calcium hydroxide
eluviation in concrete lining

Table.1 Outline of the tunnelTunnel type
Year completed
Lining thickness
Ventilation
Design speed
Traffic configuration
Tunnel length
Geological condition (West side)
Geological condition (East side)

3.1 Overview of the tunnel and cores collected
Concrete cores were collected for analyisis from theconcrete lining of a road tunnel that had served 40years. Table 1 describes the tunnel. Two types ofconcrete cores were analyzed: concrete cores ol2

Road tunnel
1957
Arch 45cm
JF Mechanical ventilation (vertical)
50km/hTwo lanes
1220m
Schist (140m, highly to moderate weathering)
Granite (120m, highly to moderate weathering)lected from a part without any damage such ascracking (hereinafter, the “sound cores”) and thosecollected from a part suffering penetration cracking
and water leakage (hereinafter, the “damagedcores”). Photos 2 and 3 show the sound cores anddamaged cores, respectively. The former showed nosign of progressive neutralization while the latterhad progressive neutralization on the crack surfaces.

A2

GroundA4
A3A1

Inner side

Inner side

Photo.2 DTG Analysis sample collection locations
for the sound cores

3.2 Analysis method
Differential thermogravimetric analysis (hereinafter,the “DTG analysis”) was employed. The samples
used for the DTG analysis consisted of the soundcores, namely A1, A2, A3 and A4 collected from apart without any neutralization (Photo.2), and thedamaged cores, namely B1, B2, B3 and B4 from apart undergoing neutralization along cracks (Photo.3).
Platinum was used for a sample cell, and theanalysis condition was established with a programming rate of 20 degrees Celsius/min and a target
temperature of 1,000 degrees Celsius. The analysis
temperature was set at 400 to 500 degrees Celsiusand 600 to 800 degrees Celsius for calcium hydroxide and calcium carbonate, respectively. The
amount of each component was obtained by molconversion9), as shown in Equations 2, 3, and 4.

GroundB2

GroundB4

B1

B3

Inner side

Inner side

Photo.3 DTG Analysis sample collection locations
for the damaged cores

TGADTA
Mass decrease

Reduction

Endoergic reaction

Calcium hydroxide content(mg)=X×(Ca(OH)2)/(H2O)=W (Eq.2)
where,
X: Mass decrease (mg)
Ca(OH)2: Molecular weight of the calcium hydroxide
H2O: Molecular weight of water

Fig.5 Example of DTG analysis results for thesound cores (A2)

TGA
DTA

Calcium carbonate content(mg)=X×(CaCO2)/(CO2)=W (Eq.3)
where,
X: Mass decrease (mg)
Ca CO2: Molecular weight of the calcium carbonate
CO2: Molecular weight of the carbon dioxide
Content percentage(%)=(W/I)×100
where,
W: Calcium hydroxide content (mg)Calcium carbonate content (mg)
I: Initial sample mass (mg)GroundReduction
Mass decrease

Endoergic reaction(Eq.4 )
Fig.6 Example of DTG analysis results for thedamaged cores (B2)
Tables 2 and 3 and Tables 4 and 5 show theamountof calcium hydroxide and calcium carbonate
with endoergic reaction temperature for the soundcores and damaged cores, respectively. As seenfrom Fig.5, the sound cores show an endoergic
reaction temperature of approximately 470 degrees
Celsius with the confirmed existence of calcium

4. Results of DTG analysis and considerations
Figures 5 and 6 show the results of DTG analysis
on the sound cores and damaged cores, respectively.
3hydroxide, an observation which was common to
all the samples, i.e. A1, A2, A3 and A4. On the other hand, as seen from Fig.6, there was no sign of anendoergic reaction between 400 and 500 degrees
Celsius in the damaged cores, which then underwent an endoergic reaction between 600 to 800 degrees Celsius with the confirmed existence of calcium carbonate. This was common to all the samples, i.e. B1, B2, B3 and B4.
These results show that at least the damaged
cores with water leakage were experiencing theeluviation of calcium hydroxide. Generally speaking, the eluviation of calcium hydroxide produces
many apertures, which accelerates crack development in concrete.

Table.2 Calcium hydroxide content (sound cores)
Endoergic reaction
temperature（℃）

Initial sample mass（mg）

A1474.15

10.07

A2472.65

10.05

A3472.72

10.02

A4468.42

10.03

ResultSample code

MedianCalcium hydroxide
Contained amount (mg)Content percentage (%)
0.23
2.32
0.19
1.84
0.20
1.97
0.12
1.23
1.84

Table.3 Calcium carbonate content (sound cores)
ResultSample code

Endoergic reaction
temperature（℃）

Initial sample mass
（mg）

A1

577.37

10.07

A2577.91

10.05

A3577.56

10.02

A4577.45

10.03

Median5. Conclusions

Calcium hydroxide
Contained amount (mg)Content percentage (%)
0.29
2.91
0.28
2.81
0.40
4.02
0.26
2.62
3.09

Table.4 Calcium hydroxide content (damaged cores)

The tests on cores collected from a concrete liningthat had served for 40 years confirmed the eluviation of calcium hydroxide in concrete with waterleakage. It is assumed that when calcium hydroxide
underdoes eluviation, the hardened cement paste inconcrete becomes porous and the cracking processaccelerated, causing the partial exfoliation of concrete. These results, however, do not constitute thenecessary and sufficient condition to prove the hypothesis about concrete lining deterioration caused
by calcium hydroxide eluviation. Other contributing
factors are also assumed to be at work, such as theautogenous shrinkage of concrete, environmental
actions, and external forces such as earthquake.
The authors are planning to develop test specimens to simulate tunnel linings to quantitatively
identify the level of calcium hydroxide eluviationand impacts of contributing factors.

Endoergic reaction
temperature（℃）

Initial sample mass
（mg）

B1

449.9010.08

B2450.4410.00

B3451.41

10.05

B4450.59

10.09

ResultSample code

MedianCalcium hydroxide
Contained amount (mg)Content percentage (%)
0.11
1.06
0.23
2.34
0.22
2.17
0.20
1.99
1.89

Table.5 Calcium carbonate content (damaged cores)
Endoergic reaction
temperature（℃）

Initial sample mass（mg）

B1

688.7810.08

B2729.80

10.00

B3705.15

10.05

B4692.91

10.09

ResultSample code

MedianCalcium hydroxide
Contained amount (mg)Content percentage (%)
0.65
6.48
1.31
13.05
0.79
7.85
0.50
4.96
8.08

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4