Journal of Advanced Concrete Technology Vol. 13, 155-162, March 2015 / Copyright © 2015 Japan Concrete Institute 155

Technical report

Effects of Improving Endurance of Concrete Structures by Wet Curing System Seishi Shiraiwa1*, Kazuto Fukudome2, Atsushi Saito1 and Toshinari Hayashi1

A selected paper of ICCS13, Tokyo 2013. Received 12 November 2013, accepted 16 February 2015 doi:10.3151/jact.13.155

Abstract There has been no such engineering method until now to enable water curing in the vertical surfaces of concrete after the removal of formworks. The Aqua curtain wet curing system has been developed with the purpose of supplying the suf-ficient water on vertical surfaces of concrete. Aqua curtain prevents rusting of reinforcing bars by making the surface of concrete denser. In this way, Aqua Curtain also contributes to realize more resistant and long-life concrete structures. In other words, implementation of this system leads to improve the endurance and economy of concrete structures by re-ducing the consumption of resources.

1. Introduction

In the fabrication of concrete constructions, curing is one of the most important processes. In order to obtain the desired concrete performance and to ensure dura-bility, in terms of strength and mass change ratio, it is important to hydrate the cement properly. Therefore, it is crucial to maintain the moisture level required for the period of hydration.

To guarantee this, the authors have developed a novel wet curing system, called Aqua Curtain (AC), which enables the supply of curing water from the outside to the interior of vertical surfaces of concrete structures after the removal of the formworks (Furu-kawa et al. 2011, 2012).

This paper reviews the Japanese specifications relevant to the curing process and provides an overview of the AC system. In addition, we present the findings of a quantitative test conducted to ascertain the effects of the curing method. Finally, examples of system’s application to actual structures are described.

2. Overview of the AC curing system

2.1 Status of system development Wet curing, which not only prevents moisture dissipa-tion but also actively supplies water from outside, has a significant effect on the performance of concrete. However, while it is easy to supply moisture to a horizontal surface, it is difficult to conduct perfect wet curing at the construction site, particularly the applica-tion of wet curing, to vertical and inclined surfaces after the formwork has been removed. The developed AC

system can form a water film on the surface of concrete structures, including elements such as vertical walls and internal surfaces of tunnel linings, and ensures that an appropriate moisture condition is maintained.

Figure 1 shows the Neville’s categorization modi-fied to include the AC curing method.

2.2 Construction technique for AC curing 2.2.1 System configuration This system consists of a curing sheet, suction equip-ment, and water supply equipment. A scheme of the system when AC is applied to a vertical wall is shown in Fig. 2. The system evacuates air from the gap be-tween the curing sheet and the concrete surface using the suction equipment to create decompression, which brings the curing sheet in close contact with the con-crete surface. Water is then supplied between the con-tacting surfaces to form a water film over the concrete surface. 2.2.2 Construction process The curing construction process is described below. • The formwork, support members, and binding wires

are removed. • A water supply hose is positioned at the upper part of

the concrete surface and a curing sheet is temporarily attached.

• Suction ports are positioned at the bottom of the curing sheet at intervals of approximately 4 m.

• The suction equipment and the suction pipe are connected.

• After ensuring that the edges of the curing sheet are airtight, the suction fans are activated.

• A supply of curing water from the water supply hose connected to the water supply pump is initiated to start the wet curing process. In this way, the watertight arrangements secure a

layer of water is always covering the concrete surface.

2.2.3 Adjustment of water supply Water supply is adjusted so that a specified amount of

1Civil Engineering Division, Hazama Ando Corporation, Tokyo, Japan. *Corresponding author, E-mail: shiraiwa.seishi@ad-hzm.co.jp 2Ishikawa National College of Technology, Ishikawa, Japan.

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 156

water is provided intermittently from the start to the end of the curing process in accordance with the water absorption rate of the concrete. Points to note are de-scribed below. • The amount of water supply required is determined

by ascertaining the amount required to wet the entire surface of the applicable concrete area.

• Required amount of water for curing will be de-creased as time passes, as shown in Fig. 3. Effective water supply period is about 7-14 days after curing starts (Shiraiwa et al. 2010).

2.3 Main equipment 2.3.1 Curing sheet Two types of materials are used for the curing sheet, as shown in Fig. 4: material having an uneven surface, used for the decompression section, and that having a smooth surface, used for the airtight section. The fea-tures of the curing sheet are described below. • Bubble wrap sheet: In order to expand the decom-

pression area, it is important to secure an adequate air flow.

• Unwoven fabric: The water holding capability of the sheet can be improved by attaching hydrophilic unwoven fabric to the surface of the concrete cov-ered with the bubble wrap.

2.3.2 Electrical equipment A turbo fan (0.4kW) is used for stabilizing the suction rate for about one week. Curing a surface of 60m long and 2m high for one week has a maximum electricity consumption of 67kWh.

Curing Wet curing

Membranecuring

Water spraying, flooding, Ponding

Covering the concrete with wet sand or earth, sawdust or straw

Periodically-wetted burlap or cotton mats

Soaking hoses(On inclined or vertical)

Covering the surface of the concrete with overlapping polyethylenesheeting , with reinforced paper, with leaving formworks

Spray-applied curing compounds which form membrane

Ingress of water into concrete by AQUA CURTAIN(On inclined or vertical)（Method of curing in which a bubble wrap sheet is adhered to the concrete surface using decompressionand a film of water is formed on the surface of the concrete ）

* Italics indicate the amendments made by the authors to Neville’s paper.Architectural Institute of Japan 2000,and Neville 2002

Fig. 1 Categories of curing.

【Suction equipment】

【 Curing sheet 】 Curing location【Water supply equipment】

(Airt

ight

sect

ion)

Water supplypump

Curing sheet

（Decompression section）

Suction fan

Water collectiontank

Port

Port

Air discharge

Port

Upper

Water supply hose

Lower

Fig. 2 Schematic diagram of AC system.

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orpt

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oncr

ete

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H

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BB+E

Fig. 3 Amount of water supply required.

Con

cret

e

Atm

osph

eric

pre

ssur

e

Bubble wrap sheet

Suction pipe

Suction port

Water supply hose

Flow ofwater and air

Unwoven fabric

Curing water

Fig. 4 AC system section at curing location.

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 157

A pump (0.59kW) is used for supplying curing water for about one week, 4 times per day, 5 minutes per one time in the same situation. The maximum amount of electricity consumption is 1.4kWh.

With minimal energy consumption, AC maximizes concrete endurance.

2.3.3 Water supply hose The water supply hose should be approximately 70–100 m long. 3. Effects of AC wet curing

3.1 Purpose and overview of tests To ascertain the effects of wet curing, its impact on the development of the compressive strength of normal Portland cement is investigated. It was found that wet curing improves the compressive strength according with the application period (Fukudome et al. 2010). Furthermore, not only strength but also mass change ratio related to durability is important in attaining the expected performance of concrete.

Okazaki et al. conducted strength tests, water ab-sorption tests, and air permeability tests on specimens created under different curing conditions, and per-formed a quantitative assessment of the effect of dif-ferent curing methods on the strength and mass change ratio of concrete (Okazaki et al. 2006).

The tests results showed that there is a clear correla-

tion between hydration rate and compression strength, but the mass change ratio obtained from the water absorption test and air permeability test varies accord-ing to the differences in curing methods.

On the basis of these results, Okazaki et al. con-cluded that with respect to mass change ratio, the permeability of concrete is determined mainly by the continuity of the pore structure rather than the number of pores and the hydration rate.

In this study, in order to investigate the effects of wet curing on durability, freeze-thaw resistance, carbona-tion resistance, and pore size distribution tests were conducted to evaluate mass change ratio quantitatively.

The mix proportions for the concrete used are shown in Table 1. The cement used included normal Portland cement (N) and Portland blast furnace slag cement (BB). In addition to simulations run using general structures, a simulated case (BT) using tunnel lining concrete that had the formwork removed at an earlier stage than usual and on which it is difficult to apply wet curing was conducted. The target values for slump and air volume at the time of pouring the concrete were 8 ± 2.5 cm and 4.5% ± 1.5%, respectively.

Details of the curing conditions are shown in Table 2. Neville’s categorization, introduced in Fig. 1, is usually employed in Japan. However, it is still not clear whether sheathing board retention can be considered as wet curing. Therefore, for simplicity, it is expressed as membrane curing in Table 2.

Table 1 Mix proportions. Unit content (kg/m3)

Fine aggregate Symbol W/C(％) s/a (％) Water W Cement C S1 S2 Coarse

aggregate G Admixtures

N 55.0 43.1 157 286 561 241 1073 2.86 BB 55.0 43.4 153 279 567 243 1073 2.97 BT 60.0 46.5 164 274 600 275 1001 2.92

*Cement Type N: Ordinary portland cement, BB and BT：Blast-furnace slag cement(TypeB)

Table 2 Curing conditions. Categories Symbol Method of curing

Ponded in water W

Curing for the study of concrete performance standards -Remove the formwork after 2 days of material aging and then conduct the standard water curing (20ºC). -After completing curing (28 days), conduct air curing.

A1 A2 Wet curing A3

Verification of the AC curing effects -Remove the formwork after 3 days of material aging (15 h for BT) and then conduct AC curing. -Curing period (N, BB: 1/2/3 weeks, BT: 1/2/3 months) -After completing curing, conduct air curing.

S

JSCE curing period for actual structures (Recommended by JSCE*) -After the curing period described in the specifications, remove the formwork and conduct air curing.-Curing period (N: 5 days, BB, BT: 7 days) -After completing curing, conduct air curing.

Sheathing curing (Kind of

Membrane Curing)

P

Curing with a shortened curing period (60% of the JSCE curing period) -Shorten the curing period to 60% of the specifications and conduct curing. -Curing period (N: 3 days, BB: 4.2 days, BT: 15 h) -After completing curing, conduct air curing.

*JSCE: JAPAN SOCIETY OF CIVIL ENGINEERS

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 158

3.2 Effect on pore structure 3.2.1 Pore size distribution according to cur-ing conditions The compressive strength and pore size distribution were measured using mercury intrusion porosimetry to ascertain the effect of AC curing on the pore structure of concrete. For this measurement, in order to eliminate the influence of coarse aggregates, a mortar specimen (φ5 mm × 10 mm) prepared in laboratory crushed to a size smaller than 5 mm and wet screened using a 5 mm sieve was used as a sample.

The measurement results for pore size distribution are shown in Fig. 5. The pore size distribution for each type of cement shifted to the fine-size pore side, be-coming a dense pore structure, as the AC curing period was extended.

The pore size from sheathing curing(S,P) using ce-ment N and cement BB peaks at around 100 nm in distribution, and has a rough pore structure compared

to AC curing and water curing (W,A1 to A3). Moreover, with the tunnel specimen (BT), which had an extremely short curing period (P), the pore size is close to 1000 nm, and this trend is noticeable.

3.2.2 Curing conditions and pore volume In order to conduct a quantitative evaluation of the differences in pore size distribution according to the different curing conditions and cement types, the effect of curing conditions on the pore volume of a rough-size pore (pore sizes: 10 nm and 50 nm) was studied. The relationships among the curing conditions, the total pore volume, and the pore volume with pore sizes of 10 nm and 50 nm or more are shown in Fig. 6.

The effect of the curing conditions on the total pore volume was minor, but a significant difference oc-curred in pore volume when the pore size was 10 nm, 50 nm, or more. In particular, the pore volume for the rough-pore size increased when sheathing curing was

Cement:N

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dＶ

(D)/

dlo

g（D

）

W S PA1 A2 A3

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dＶ

(D)/

dlo

g（D

）

WPA1

Fig. 5 Pore size distribution.

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Curing conditions

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l/g）

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Total pore volume

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Curing conditions

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e（m

l/g）

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Cemnet: BBT

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Pore

volu

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Fig. 6 Relationship between the curing conditions and the pore volume.

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 159

used, while it clearly decreased for AC curing as the curing period was extended. Therefore, it was found that there is a clear correlation among pore structure, curing period, and curing conditions.

3.2.3 Correlation between pore volume for each pore size and compressive strength The correlation between the calculated pore volume for each pore size and the compression strength was in-vestigated.

The relationship between pore volume and com-pression strength is shown in Fig. 7. A correlation with each pore volume is apparent, but the degree of corre-lation increases for pore volumes with a pore size of 10 nm or more. It is thought that the differences in the pore structure in accordance with the curing conditions, described earlier, are due to the hydration level.

3.3 Effects on freeze-thaw resistance A freeze-thaw resistance test was conducted using normal Portland cement according to Method A of JIS A 1148(Method of test for resistance of concrete to freezing and thawing), which is comparable to ASTM C 666(A). The material age at the start of the test was 28 days. A freeze-thaw resistance test on a specimen that was subjected to air curing, but not water curing, was conducted immediately after measuring the initial value when the specimen was taken from a tempera-ture-and humidity-controlled room without having absorbed any water. Measurements were performed once every 30 cycles up to 300 cycles.

The relationship between freeze-thaw cycles and relative dynamic modulus of elasticity is shown in Fig. 8.

The symbols in Fig. 8 indicate the curing methods shown in Table 2. The reduction in the relative dy-namic modulus of elasticity associated with the freeze-thaw cycles decreases with the curing period. This is because the specimen becomes progressively drier with decreasing curing period; the degree of saturation is low and the specimen is less susceptible to deterioration resulting from freeze thaw.

The calculation results of the dynamic modulus of elasticity based on the number of resonant vibrations and the test specimen mass according to JIS A 1127(Method of test dynamic modulus of elasticity, rigidity and Poisson’s ratio of concrete by resonance vibration) are shown in Fig. 9.The relative dynamic modulus of elasticity varies depending on the curing conditions, and this leads to a difference in the concrete performance. The dynamic modulus of elasticity in-creases in the initial freeze-thaw cycles as the curing period is shortened. This is because hydration of the test specimen, which initially has insufficient hydration, progresses.

The relationship between freeze-thaw cycles and mass change ratio is shown in Fig. 10. The mass at the time of pouring the concrete is used as the reference for

the mass change ratio. The mass of test specimens that were not subject to water curing increased because of the absorption of water over 30 cycles, and the mass change ratio was almost the same as with water curing. Therefore, the test specimen can be considered to be water saturated.

Differences in mass change were generated after 30 cycles in accordance with the curing conditions. At 300 cycles, the mass change ratio of sheathing curing (S, P) was greater than that of water curing and AC curing (A1 to A3) by 1.5%–4.5%. We can thus say that the freeze-thaw resistance of sheathing curing is lower than that of AC curing.

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pre

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tren

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ith t

he

mat

eria

l ag

e of 91 d

ays　

（N

/m

m2）

Diameter 50 nm <nm

Diameter 10 nm <nm

Total pore volume

Fig. 7 Relationship between pore volume and compres-sion strength.

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Rela

tive d

ynam

ic m

odulu

s of

ela

stic

ity (%)

W S PA1 A2 A3

Fig. 8 elationship between freeze-thaw cycles and rela-tive dynamic modulus of elasticity.

303234363840424446

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Freeze-thaw cycles (times)

Dyn

amic

modu

lus

of el

astici

ty

(kN

/m

m2)

W S PA1 A2 A3

Fig. 9 elationship between freeze-thaw cycles and dy-namic modulus of elasticity.

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 160

The mass changes linearly after 30 cycles of freeze thaw; therefore, the mass-change speed according to the freeze-thaw cycles is calculated using linear re-gression.

Figure 11 shows the relationship between curing period (material age after completing curing) and mass change speed. The mass change speed decreases con-siderably with increasing curing period. The effect of AC curing is significant, and the mass change de-creases at the same rate as when water curing was conducted over 28 days.

3.4 Effects on carbonation resistance An accelerated carbonation test according to JIS A 1153 (Method of accelerated carbonation test for con-crete), which is examined in the environment of 5% of CO2, was conducted to ascertain the effect on im-provement in the quality of the surface layer of con-crete by wet curing.

To reproduce the five curing methods (A1 to A3, S, P) shown in Table 2, large test specimens (thickness 0.3 × height 1.2 × length 7.2 m) were fabricated. Core specimens were taken from these specimens with a material age of 26 weeks for use in accelerated car-bonation tests up to an accelerated period of 13 weeks.

The carbonation depth caused by the differences in the cements and the curing methods is shown in Fig. 12. Our findings indicate that because the carbonation depth for AC curing is less than that for sheathing curing, the structure of the concrete surface becomes denser due to adequate water absorption.

4. Examples of On-Site Applications

4.1 Construction example for a tunnel In most examples of lining concrete application, the formwork is removed at an early stage, 12 to 20 hours after the concrete has been poured (Japan Society of Civil Engineers 2006.).

Moreover, because of the misunderstanding as to whether the temperature in a tunnel is stable and the ambient conditions are sufficiently wet, special curing is not conducted. However, a study conducted by Baba

et al. showed that a humidity of 60%-70% in a tunnel is an inadequate curing environment for concrete (Baba 2009).

In addition, a tunnel is one of the most difficult structures to be used for wet curing. This is because tunnels have an arch shape and the formwork is re-moved at an early stage. Therefore, anchors or other devices for securing curing equipment cannot be used.

With respect to this, we present here some examples of AC applications in tunnel construction. Figure 13 describes the scheme of the application of AC in the construction of a tunnel. To get this configuration, the construction process is slightly modified. First, the water supplying tube is attached to the sheets. Then, this system is erected by using a system of multi-frames. After that, the suction pipes are connected to the suc-tion ports and the air is extracted. In this way, the sys-tem becomes watertight. Finally, the water supply starts.

As shown in Fig. 14, the configuration adopted fits the geometric characteristics of an arch tunnel.

4.2 Application examples for other structures This curing method can be applied to other types of concrete structures. In Japan, up to now, there have been many examples of application, such as the con-

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Mas

s cha

nge

ratio

(%)

W S PA1 A2 A3M

ass

chan

ge r

atio

(%)

Fig.10 elationship between freeze-thaw cycle and mass change ratio.

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Mas

s ch

ange

spee

d (%

/cycl

e)

W S P

A1 A2 A3

Fig. 11 elationship between wet curing period and mass change speed.

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Curing Condition

Acc

eler

ated

carb

onat

ion d

ept

h(n

m)

NBBBT

S P A1 A2 A3

*Cement Type N:Ordinary Portland cement BB and BT:Blast-furnace slag cement(TypeB)

Fig. 12 Comparison of the carbonation depth between curing conditions.

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 161

struction of bridge piers, box culvert walls, prestressed concrete tank walls, external walls of water treatment plants, and retaining side walls. Construction photo-graphs from application examples are shown in Figs. 15 and 16.

In addition, strains of drying shrinkage on real structures were measured in a wall of the vertical shaft which uses an expansive concrete. Figure 17 shows

these results. Strains due to drying shrinkage of con-crete cured by AC were smaller than those of concrete cured under plastic sheet cover (Shiraiwa et al. 2011).

5. Conclusions

In this study, the effects of AC wet curing on improving concrete durability were ascertained. The findings from

[Suction equipment]

[Water supply equipment]

Air discharge

Suction fan

Water collection tank

[Multi-frames]This frames are installed for every span for the purpose of preventing the unlikely dropping of the curing sheet.

[Water supply hose]Curing water is intermittently supplied by hose.

Suction portSuction pipe

Carriage

Cur

ing

Wat

er

Water supply pump

[Curing sheet]１施工単位あたり１枚に加

工したものを使用

200~250 ㎡/unit

Fig. 13 AC System configuration for a tunnel (example).

Fig. 14 Application to tunnel lining concrete.

Fig. 15 Application to a wall at a channel.

Fig. 16 Application to a wall at a prestressed concrete tank.

S. Shiraiwa, K. Fukudome, A. Saito and T. Hayashi / Journal of Advanced Concrete Technology Vol. 13, 155-162, 2015 162

this study are described below. Compared to sheathing curing, wet curing improves

freeze-thaw resistance and carbonation resistance. By applying wet curing, which provides adequate

moisture, the concrete surface becomes denser and the pore structure becomes the same as that obtained with water curing.

It was found that the AC system can be applied, and wet curing correctly conducted, even to sections on which it is generally difficult to conduct wet curing, such as vertical walls and arch structures, and that when applied, it exhibits superior practicality.

On the basis of the above mentioned findings, AC curing prevents the rusting of reinforcing bars by making surface concrete dense, realizing a good and long-life concrete structures, and leads to improve the endurance of concrete structures by reducing and pre-venting the consumption of resources.

References Architectural Institute of Japan, (2000). “Recommen-

dation for practice of hot weather concreting.” 106-116 (in Japanese)

Baba, K., (2009). “Study on the tunnel environment under construction and humidity change of lining concrete.” Journal of JSCE, 742, VI-60, 27-35 (in Japanese)

Fukudome, E., Furukawa, K. and Shono, (2011). “Study on the estimation method of strength development of concrete under vvarious moisture curing conditions.” Journal of Japan Society of Civil

Engineers, Ser. E2 (Materials and Concrete Structures) 67(1), 18-27. (in Japanese)

Furukawa, Y., Suzuki, M. and Mitai, Y., (2011). “Development of moisture curing system for tunnel concrete lining.” 12th ISRM International Congress on Rock Mechanics, 1675-1678.

Furukawa, Y., Fukudome, K. and Mitani, Y., (2012). “Effects of improving durability of concrete by AQUA CURTAIN curing and its application.” Journal of Civil Engineering and Architecture, 6(5), 574-585.

Japan Society of Civil Engineers, (2006). “Standard Specifications for Tunneling-2006, [Mountain Tun-nels].” 94-96.

Neville, A. M., (2002). “Properties of concrete.” 4th and Final Edition Standards updated to 2002, 323-326.

Okazaki, S., Yagi, T., Kishi, T. and Yajima, T., (2006). “Difference of sensitivity due to curing condition on strength and permeability.” Cement Science and Concrete Technology, 60, 227-234. (in Japanese)

Shiraiwa, S., Furukawa, Y. and Shono, A., (2010). “Study on the amount of water absorption rate and the period of wet curing of concrete after the removal of concrete form.” JSCE 65th Annual Meeting, V-674, 1347-1348. (in Japanese)

Shiraiwa, S. and Saito, A., (2011). “Study on evaluation method of the effect of curing at the real structure.” JSCE 66th Annual Meeting, V-280, 559-560. (in Japanese)

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in o

f dry

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アクアカーテン養生

Finish curing withwrap sheet

Finish curingwith AC

127μ

－

71μ

56μ

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Curing with bubble wrap sheetAC

Fig. 17 Strain of drying shrinkage measurement results for wall of the vertical shaft.