study on mechanism of strength distribution development

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O R I G I N A L A R T I C L E

Study on mechanism of strength distribution development

in vacuum-dewatered concrete based on the consolidationtheory

Shigemitsu Hatanaka Hiroki Hattori

Eisuke Sakamoto Naoki Mishima

Received: 5 June 2009 / Accepted: 5 January 2010 / Published online: 21 January 2010

RILEM 2010

Abstract The strength and hardness of a concrete

slab surface are considered to be significantly affected

by concrete bleeding. Vacuum dewatering is reported

to be quite effective in imparting high density and

strength. However, in Japan, in contrast with concrete

work in civil engineering applications, concrete work

in the field of building construction has not been

successfully treated by this method. In an earlier

report, the authors pointed out the strong relationship

between strength distribution and density distribution

in vacuum-dewatered concrete, both of which grad-ually decrease from the top surface to a depth of about

15 cm. The main purpose of the present study is to

discuss the mechanism of the occurrence of such

distribution of strength and density, based on consol-

idation theory. In an experiment, pore water pressure

distribution in concrete is measured by means of an

original measuring system. The results of the exper-

iment confirm that the consolidation theory is quite

effective in explaining the internal properties of

vacuum-dewatered concrete as well as those of

press-dewatered concrete. A prediction method for

the strength improvement of concrete by vacuum

dewatering is also discussed. It was considered likely

that pore water pressure distribution generated byvacuum dewatering could be attributable to the

influences of capillary tension and viscous resistance.

This mechanism was verified by model experiment.

Keywords Vacuum dewatering methodPress dewatering Consolidation theory Pore water pressure distribution Concrete

1 Introduction

On the surface of a concrete floor slab, a weak layer

is inevitably formed due to bleeding. In many cases,

the quality of finishing work depends on the quality

of the surface layer. The vacuum dewatering method,

which is a construction technique that dramatically

improves the strength and durability of hardened

concrete by removing excess water from the concrete

S. Hatanaka N. MishimaDivision of Architecture, Graduate School of Engineering,

Mie University, 1577 Kurimamachiya-cho, Tsu, Mie,

Japan

e-mail: [email protected]

N. Mishimae-mail: [email protected]

H. Hattori (&)

Department of Design for Contemporary Life, Gifu City

Womens College, 7-1 Hitoichibakitamachi, Gifu, Gifu,

Japan


E. Sakamoto

Mie Prefectural Center of Constructional Technology,

3-50-5 Sakurabashi, Tsu, Mie, Japan


Materials and Structures (2010) 43:12831301

DOI 10.1617/s11527-010-9580-1


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with a vacuum pump immediately after concrete

emplacement, is one of the techniques that solve this

problem [111]. The authors have developed a new

vacuum dewatering method and have conducted a

series of experiments [12] in order to contribute to

steady ongoing technological improvements in the

vacuum dewatering method by correcting the knowndefects of the conventional method.

As shown in Fig.1, the greatest improvement

achieved by the vacuum dewatering method is

increased compressive strength in a higher layer.

However, the mechanism of this phenomenon is not

yet known in detail.

Therefore, this paper seeks to clarify the mecha-

nism of compressive strength distribution and density

distribution in concrete induced by vacuum dewater-

ing, by applying the consolidation theory used in the

field of soil engineering, and proposes a technique of

estimating improvement of concrete compressive

strength by vacuum dewatering.

2 Consolidation theory

2.1 Consolidation phenomenon

Figure2 illustrates the concept of the consolidation

phenomenon. The target specimen is divided into

solid and liquid phases, which are assumed to be

incompressible. The consolidation phenomenon is

that occurs when the liquid phase is discharged with

the passage of time under consolidation pressure,

thereby reducing volume and increasing density. Bydecompressing pores in concrete, the vacuum dewa-

tering method produces atmosphere-equivalent pres-

sure on the top surface of a slab where fresh concrete

is consolidated by atmospheric pressure and surplus

water is discharged.

2.2 Press dewatering and vacuum dewatering

Figure3illustrates the concept of stresses by press

dewatering and vacuum dewatering [13] in situations

where distribution of pore water pressure by the depthof specimen is not assumed. Judging from the figure,

if the consolidation pressure is equal to the degree of

decompression by vacuum dewatering, the effective

stress at the end of dewatering is the same. Based on

this result, in the sections that follow the authors

attempt to analyze the vacuum dewatering process as

a phenomenon similar to consolidation by press

dewatering.

10 20 30 40 50

Compressive strength MPa

Layer 1

Layer 2

Layer 3

Layer 4

Upper surface of slab

No processing

Vacuum dewatering

Fig. 1 Compressive strength distribution (slab thickness:

24 cm)

hi

S

fi = Vi/ Vs

Vi

Mi

Vs

Ms

hsSolid phase

Liquid phase

Consolidationp

ff = Vf / Vs

Vs

Ms

hs

Vf

Mf

hf

Liquid phase

Solid phase

(a) (b)

Fig. 2 Concept of

consolidation phenomenon.

a Before loading (subscript

i),bafter loading (subscript:

f).Notes: Vvolume (cm3

),

Mmass (g),h height (cm),

Samount of compaction

(cm), fvolume ratio,

pconsolidation pressure

(MPa)

1284 Materials and Structures (2010) 43:12831301


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2.3 Consolidation equation

In the vacuum dewatering method, water permeates

only through the top surface, and slab thickness is

generally small enough, relative to the area of a filtering

mat. Therefore, the one-dimensional consolidation

theory is applied here. Since the purpose of the present

study is to quantitatively clarify the generating mech-

anisms of density distribution and compressive strength

distribution in concrete, the authors decided to useEq.1 as the most basic consolidation equation express-

ing the relationship between compaction and time. The

unit of timetin the equation is seconds (s) because the

processing time of this proposed method is about 300 s

[12]. Equation1 applies to a homogeneous specimen

where the effects of changes in the specimen weight and

excessivechanges to layer thickness are not considered.

Equation1is called Mikasas equation of consol-

idation. By solving this equation, the relationship

between the degree of consolidation Us and the timefactorTvis known and can be drawn as single curve,

as shown in Fig.4 [14]. Where, the degree of

consolidation is the dimensionless amount of com-

paction S/Sf (S: the amount of compaction, Sf: the

final amount of compaction) and the time factor Tvis

a dimensionless time. That is defined by Eq. 2 for

layer thickness h (= H/2) in the case of single-side

permeation. If the final amount of compaction Sfand

the coefficient of consolidation cv are known, the

relationship between the amount of compaction Sand

the time tcan be derived from Eq.2and Fig.4.The coefficient of consolidation cv in Eq.1

dominates the consolidation speed. Since this study

covers only up to the end of consolidation, the

authors decided to calculate the coefficient using the

curve ruler method. For single-side permeation, the

curve ruler method expresses the coefficient of

consolidation cv by Eq.3[15].

oe

ot cv

o2e

oz2 1

wheree is the compressive strain, tis the time (s), cvis the coefficient of consolidation (cm2/s), and z is the

layer depth (cm).

Tvcv t

h2 2

where Tv is the dimensionless time factor, t is the

time (s), and h is the layer thickness in the case of

single-side permeation (cm).

(a)

(b)

u0 0

Stress

Before loading

(self-weight consolidation)

D

epth

u0 0

Effective stress 0+

Stress

Consolidationpressure

End of dewatering

D

epth

u0 0

Stress

Depth

Before processing

(self-weight consolidation)

0

Stress

Atmospheric

pressure

Depth

Effective stress 0+

End of dewatering

0

Fig. 3 Stresses at press dewatering and vacuum dewatering.a

Press dewatering, b vacuum dewatering.Notes:r effective stress

at theend of dewatering (MPa),r0 effective stress before loading

or processing (MPa), u0 pore water pressure before loading or

processing (MPa), Drincrease in effective stress (MPa)

0

20

40

60

80

1000.001 0.01 0.1 1

Time factor Tv[Logarithmic scale]

10

Us(=S/Sf)

(%)

Fig. 4 Relationship between degree of consolidation Us and

time factor Tv

Materials and Structures (2010) 43:12831301 1285


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cv0:197 h2

t503

where h is the average height of specimen (cm), and

t50 is the time when the theoretical degree of

consolidation becomes 50% (s).

3 Experiment concerning density distribution

(Experiment 1)

3.1 Outline of experiment

In Experiment 1, the density distributions of vacuum-

dewatered and press-dewatered mortar and concrete

were measured to verify the applicability of the

consolidation theory.

3.1.1 Experiment using mortar

Table1 lists the experimental factors and measure-

ment items, and Table2 explains the mixture

process. The experimental factors are the dewatering

method and consolidation pressure. Press dewatering

and vacuum dewatering were assumed for the

dewatering method, and the consolidation pressure

p by press dewatering was set to two levels,

0.05 MPa (about 1/2 of the atmospheric pressure)

and 0.1 MPa (equivalent to atmospheric pressure).

Figure5shows the press dewatering experimental

setup for this experiment. In order to make the

conditions equal to those for vacuum dewatering,single-side permeation from the top surface was

assumed and a mat used for vacuum dewatering was

used for filtration. Figure6shows the vacuum dewa-

tering experimental setup. To ensure a vacuum, the top

surface of each specimen was covered with a vinyl

sheet. The degree of vacuum, as measured with a

vacuum gauge, was about 95%. Specimen size was u

100 9 120 mm for both press dewatering and vacuum

dewatering. Processing was started when bleeding was

near completion (a specimen prepared separately was

measured 120 min after mixing). Immediately beforethe processing, the bled water was sampled by means

of a syringe. As in the proposed method, processing

was continued for 300 s [12].

Figure7a shows a schematic of a core specimen.

Three core specimens of u 25 9 25 mm were

sampled from each specimen. Density was measured

at the material age of about 3 weeks.

Table 1 Experimental factors and measurement items

(Experiment 1: Mortar)

W/C

(%)

Dewatering

method

p (MPa) Measurement item

60 Press dewatering 0.05 Discharged water

by bleeding

0.1 Dewatering amount

by processing

Vacuum dewatering Relationship between

compaction and time

No processing Density distribution

(/ 25 mm core)

W/Cwater/cement ratio, p consolidation pressure

Table 2 Mix proportions (Experiment 1: Mortar)

W/C (%) s/m (%) Unit weight (kg/m3

) FL (mm) Air (%)

W C S

60 50 327 546 1300 266 1.4

W/C water/cement ratio, s/m sand volume/mortar volume,

W water, C cement, Ssand, FLmortar flow, Airair volume

Displacement gauge

Weight

Frame

Filter mat

Drain pipe

SpecimenScale

Fig. 5 Press dewatering experimental setup (Experiment 1:

Mortar)

Drain pipe

Specimen

Vacuum

pump

Scale

Drain trap

Vacuum gaugeDisplacement gauge

Frame

Sealing vinyl sheet

Filter mat

Fig. 6 Vacuum dewatering experimental setup (Experiment 1:

Mortar and concrete)



5/19

3.1.2 Experiment using concrete

Table3 lists the experimental factors and measure-

ment items, while Table4gives the mix proportions.

The experimental factors are the same as for mortar.

The consolidation pressurepby press dewatering wasset to two levels of 0.1 MPa (equivalent to atmo-

spheric pressure) and 0.5 MPa, which is even greater.

For the press dewatering experiment, dewatering

from the top surface was assumed. A load plate was

placed on a permeable mat covering a specimen, and a

universal tester was used for loading. The setup for the

vacuum dewatering experiment is as shown in Fig.6.

The measured degree of vacuum was about 85%.

Specimen size wasu 150 9 180 mmfor both the press

dewatering and vacuum dewatering experiments.

Processing was started when bleeding was near

completion (90 min after mixing). Bled water was

sampled by means of a syringe. The processing was

continued for 300 s.

Figure7b shows a schematic of a core specimen.

Three concrete core specimens of u 509

50 mmwere sampled from each specimen. Density was

measured at the material age of about 3 weeks.

3.2 Experimental result and discussion

3.2.1 Relationship between compaction and time

Figure8 shows the relationship between compaction S

and time t. As shown in the figure, higher consolidation

pressure during press dewatering produces a greater

50 mm

50mm

180mm

50 mm

150mm

25 mm

25

mm

120mm

100mm

25 mm

(a) (b)

Fig. 7 Core specimens

(Experiment 1). a Mortar,

b concrete

Table 3 Experimental factors and measurement items (Experiment 1: Concrete)

W/C(%) Specimen shape and size (mm) Process Processing

time (s)

p (MPa) Measurement item

65 Cylindrical / 150 9 180 Press dewatering 300 0.10

1000 0.50 Discharged water by bleeding

Vacuum dewatering 300 Dewatering amount by processing

No processing Density distribution (/ 50 mm core)

Box-shaped 300 9 460 9 180 Press dewatering 1000 0.50

No processing

W/Cwater/cement ratio, p consolidation pressure

Table 4 Mix proportions (Experiment 1: Concrete)

W*/C(%) s/a (%) Unit weight (kg/m3

) SP/C (%) SL(cm) Air (%)

W* C S G

65 56 185 285 993 801 0.9 19.0 3.7

W*/C watercement ratio [W*(water ? superplasticizer)], s/a sand aggregate ratio, Ccement, Ssand (coarse grain ratio = 2.98),

G coarse aggregate, SP superplasticizer, SLslump, Airair volume



6/19

amount of compaction and faster convergence. Vac-

uum dewatering (degree of vacuum: about 95%)

produces a smaller amount of compaction than press

dewatering. In Fig. 8, the white circle represents the

theoretical value of press dewatering. To calculate a

theoretical value, an experimental value is entered for

the initial amount of compaction (t= 0.1 s). Regard-ing the tendency of compaction to increase until the

final stage, the experimental and theoretical values

matched comparatively well at both 0.05 and 0.1 MPa.

Therefore, the consolidation theory is considered

applicable to the mortar dewatering process. The

experimental and theoretical values also matched

comparatively well in press dewatering with bottom-

surface permeation [16].

3.2.2 Density distribution

Figure9a, b shows the measured density distributions

of mortar and concrete specimens, respectively.According to Fig.9a, which shows mortar density

distributions, the densities of the press-dewatered

specimens (0.05 and 0.1 MPa) are greater than that of

the non-processed specimen, and the gradients in the

depth direction are about equal to that of the non-

processed specimen. Compared with the 0.05 MPa

specimen, the 0.1 MPa specimen exhibits an almost

proportional increase in density at any depth. There-

fore, as compared with the non-processed specimen,

the dewatering amount of the press-dewatered spec-

imen at each layer is estimated to be almost constantin the depth direction. Thus, the consolidation theory

is considered applicable.

The density distribution of the vacuum-dewatered

specimen is about equal to that of the non-processed

specimen at the bottom layer and about equal to that

of the 0.1 MPa specimen at the top layer. Therefore,

vacuum dewatering is considered to generate depth-

wise pore water pressure distribution.

According to Fig.9b, which shows concrete density

distributions, the densities of the press-dewatered

specimens (0.1 and 0.5 MPa) are almost equal, and

mm

0

1

2

3

4

5

60.1 1 10 100 1000

Time t(s) [Logarithmic scale]

AmountofcompactionS

Press dewatering (0.1MPa)

Solid line Measured value

Theoretical value

Vacuum dewatering

Press dewatering (0.05MPa)

Fig. 8 Amount of compaction S and time t (Experiment 1:

Mortar)

0

20

40

60

80

100

1202.20 2.25 2.30 2.35 2.40

Density g/cm3

Press dewatering 0.1MPa


Vacuum dewatering

No processing

0

30

60

90

120

150

180

Specimendepth

(mm)


Press dewatering 0.1MPaVacuum dewatering

No processing

(a) (b)

Specimendepth

(mm)

2.20 2.25 2.30 2.35 2.40

Density g/cm3

Fig. 9 Density distribution

(Experiment 1). a Mortar,

b concrete



7/19

greater than those of the non-processed specimen. This

indicates that consolidation reached the limit at about

0.1 MPa. Since the density distributions are constant in

the depth direction, the consolidation theory is con-

sidered as equally applicable for concrete as for mortar.

As in the case of mortar, the density distribution of

the vacuum-dewatered specimen is about equal tothat of the non-processed specimen at the bottom

layer and that of the 0.1 MPa specimen at the top

layer. Therefore, vacuum dewatering is considered

likely to generate depth-wise pore water pressure

distribution.

4 Experiment concerning pore water pressure

(Experiment 2)

Judging from the experimental results in Sect. 3.2,vacuum dewatering may generate pore water pressure

distribution in the depth direction of concrete, as

shown in Fig.10. In Experiment 2, the pore water

pressure distribution in vacuum-dewatered concrete

was measured and the mechanism of density distri-

bution obtained by experiment is discussed by

reference to the measurement result. Pore water

measurement using a sealed vessel proved that the

pore water pressure estimate was reliable [17].

4.1 Outline of experiment

Table5 lists the experimental factors, and Table6

gives mixing proportions. Figure11 shows a sche-

matic of the specimen vessel. The small pore water

pressure gauge depicted in Fig.11 was developed

by the authors. The pressure-receiving surface is

covered with a dewatering mat. In the experiment, in

u150-mm acrylic vessels like the one shown in

Fig.11, specimens were emplaced to heights of 105

and 180 mm and processed by ordinary vacuum dewa-

tering for about 300 s. During the dewatering period, thetime histories of pore water pressure were measured.

Pore water pressure gauges were secured to piano

wire frames at depths of 10 and 50 mm for the

105-mm-high specimen and at depths of 10, 50, 90,

and 130 mm for the 180-mm-high specimen. By

burying the gauges in each specimen, the suction

pressure by the vacuum pump and the pore water

pressure in the specimen could be measured.

4.2 Experimental results and discussion

4.2.1 Mortar

Figure12shows the time histories of the pore water

pressure and suction pressure of the mortar specimen.

In the figure, the negative pressures of both pore

water and suction are shown as positive.

According to Fig.12a, the suction pressure in a

specimen subjected to vacuum dewatering immedi-

ately after casting increased quickly after the start of

processing and after 15 s became almost constant at

0.09 MPa. At the depth of 10 mm, the pore waterpressure decreased quickly after the start of process-

ing, became 0.04 MPa after 10 s, and then decreased

gradually. At the depth of 50 mm, the pore water

pressure began to decrease gradually after 50 s of

processing and became about 0.02 MPa after 300 s.

According to Fig.12b, the suction pressure and

pore water pressure of vacuum dewatering after

bleeding show similar tendencies to those of the

specimen that had been subjected to vacuum dewa-

tering immediately after specimen casting. At the

depth of 50 mm, however, the pore water pressuregradually increased immediately after the start of

processing. At this depth, the pore water pressure

increase start timing differs, probably because the

speed of reaching depth-wise pore water pressure

differs depending on the degree of specimen com-

paction before and after bleeding.

According to Fig. 12c, the pore water pressure of

the 180-mm-high specimen showed a similar small

increase at depths of 90 and 130 mm. This fact is

e u

Effective stress

Elapsed time

pa0Pressure

Depth

e

0Pressure

- pa

Elapsed time

Depth

u

Pore water pressure

Fig. 10 Predicted pore water pressure distribution (isochrone).

Notes: u pore water pressure (MPa), re effective stress (MPa),

Pa suction pressure (MPa)



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consistent with the findings from a past study thatindicated the dewatering effect reaches a depth of

about 15 cm [12].

4.2.2 Concrete

Figure13 shows the time histories of concrete pore

water pressure and suction pressure.

According to Fig.13a, b, the time history of

concrete tends to decrease gradually at the depth of

10 mm, compared with that of mortar. This is mainlybecause, when compared with mortar, concrete is

denser, having a smaller unit water content and a

greater aggregate ratio. These factors make the

conduction of pore water pressure difficult.

According to Fig.13c, the 180-mm-high specimen

showed a quick decrease in pore water pressure at a

depth of 10 mm after about 120 s and at a depth of

50 mm after about 90 s. A possible reason is that the

air passage disturbing a decrease in pore water pressure

Table 5 Experimental factors (Experiment 2)

Factor: Specimen Specimen height (mm) Processing start timing Measurement item

series

Experiment 2 Mortar concrete 105 Immediately after casting

At the end of bleeding Pore water pressure

180 At the end of bleeding Suction pressure

Table 6 Mix proportions (Experiment 2)

W/C(%) s/m (%) Unit weight (kg/m3

) FL(mm) Air (%)

W C S

(a) Mortar

60 55 281 468 1360 212 2.6

W/C(%) s/a(%) Unit weight (kg/m3

) SP/C(%) SL(cm) Air (%)

W* C S G

(b) Concrete

65 56 185 285 993 801 0.9 19.0 3.6

W*/C watercement ratio [W*(water ? superplasticizer)], s/a sand aggregate ratio, Ccement, Ssand (coarse grain ratio = 2.98),

G coarse aggregate, SP superplasticizer, SLslump, Airair volume

Tube connectionbracket

Sealing vinyl sheet

Dewatering mat

150

10

Specimen

Pore water

pressure gauge

200

105

Acrylic vessel

(150 200 mm)

[Unit : mm]

40

150

10

402

00

180

40

[Unit : mm]

(a) (b)

Fig. 11 Sample vessel.

a specimen height 105 mm,

b specimen height 180 mm



9/19

is blocked by the dewatering process. However, the

details are unknown. Although not illustrated here,

other specimens show similar phenomena. At the

depths of 50, 90, and 130 mm, some pore water

pressure is maintained even after the end of vacuum

dewatering, probably because the mortar above the

pore water pressure gauges was sufficiently compactedby dewatering so as to prevent the release of the pore

water pressure even after vacuum dewatering.

4.2.3 Pore water pressure distribution

Figures14 and 15 show the time histories of pore

water pressure distribution for mortar and concrete,

where, the value at the depth of 0 mm surface level is

the suction pressure. According to the figures, both

mortar and concrete generate depth-wise pore water

pressure distribution at each time. The higher thelayer, the greater the pressure. The density distribu-

tions in Fig.9a, b show similar tendencies. Therefore,

it may be possible to estimate density distribution by

applying the consolidation theory while taking pore

water pressure distribution into account.

4.3 Discussion about the causes of pore water

pressure distribution

The pore water pressure is found to be less in lower

concrete during vacuum dewatering. The authorsconjecture that the water pressure distribution is

generated mainly by the influences of capillary

tension and liquid-phase viscous resistance.

The influence of capillary tension can be consid-

ered as follows. As the enlarged figures in Fig. 16

show, when vacuum dewatering occurs, pore water

pressure becomes negative. As the distance between

solid particles progressively decreases, pore water

pressure decreases, causing air bubbles to inflate and

increase their area of contact with solid particles.

Consequently, in the pores between capillary voidsthat are surrounded by solid particles, meniscus is

easily formed and generates tension within the

capillary. Once meniscus has been formed, the

gravity on water above the meniscus and the capillary

tension may become resistant to suction pressure by

vacuum processing and reduce the amount of suction

pressure that is transmitted to lower levels.

0 100 200 300

Time (sec)

Porewaterpressureorsuction

pre

ssure(MPa)

0

0.0

2

0.04

0.0

6

0.0

8

0.1

Suction pressure

Depth: 10 mm

Depth: 50 mm

(a)

0 100 200 300

Time (sec)

(b)

0 100 200 300

Time (sec)

Depth: 90 mm

Depth: 130mm

(c)

0

0.0

2

0.0

4

0.0

6

0.0

8

0.1

0

0.0

2

0.0

4

0.0

6

0.0

8

0.1

Suction pressure

Depth: 10 mm

Depth: 50 mm

Porewaterpressur

eorsuction

pressure(M

Pa)


pressure(MPa)

Fig. 12 Time histories of mortar pore water pressure (Exper-

iment 2). a Immediately after casting (specimen height: 105

mm), b at the end of bleeding (specimen height: 105 mm), c at

the end of bleeding (Specimen height 180 mm)



10/19

The influence of viscous resistance can be consid-

ered as follows. The liquid phase moves through

capillary voids surrounded by solid particles and

reaches the drain face. This may generate viscous

resistance between the capillary wall (solid phase)

and the liquid phase, consequently reducing the

suction pressure. With the advance of consolidation,

capillaries become more complicated. Therefore, the

contact area between the liquid and solid phases may

grow and increase the degree of influence of the

liquid-phase viscous resistance.

To confirm the influences of the above two factors,

the authors conducted a vacuum dewatering experi-

ment using the following model materials.

Time (sec)

Porewater

pressureorsuction

pressure(MPa)

0

0.0

20.0

4

0.0

6

0.0

8

0.1

Time (sec)


pressure(MPa)

0

0.02

0.0

4

0.0

6

0.0

8

0.1

Processing end position

0 100 200 300

0 100 200 300 400

0 100 200 300 400

Time (sec)

Porewaterpressureo

rsuction

pressure(MPa)

0

0.0

2

0.0

4

0.0

6

0.0

8

0.1

Processing end position

(c)

(a)

(b)

Suction pressure

Depth : 10 mm

Depth : 50 mm

Suction pressure

Depth : 10 mmDepth : 50 mm

Depth : 90 mm

Depth : 130 mm

Fig. 13 Time histories of concrete pore water pressure (Exper-

iment 2). a Immediately after casting (specimen height:

105 mm),b at the end of bleeding (specimen height: 105 mm),

cat the end of bleeding (specimen height 180 mm)

0

25

50

75

Pore water pressure (MPa)

Sampledepth(mm)

t=10 s

t=50 s

t=150 s

t=300 sElapsed time

0

25

50

75

100

125

150

0 0.02 0.04 0.06 0.08 0.1

0 0.02 0.04 0.06 0.08 0.1


Sampledepth(mm)

t=10 s

t=50 s

t=150 s

t=250 s

Elapsed time

(a)

(b)

Fig. 14 Time histories of mortar pore water pressure distri-

bution (at the end of bleeding). a Specimen height: 105 mm,

b Specimen height: 180 mm



11/19

4.3.1 Experimental factors and materials

Capillary tension increases as pore dimensions

between solid particles become smaller. To vary the

pore size between solid particles, the solid-phase

particle size was taken as an experimental factor.

Regarding viscous resistance, liquid-phase viscosity

was taken as an experimental factor. Table 7provides

the experimental factors and testing levels and

Table8 lists the materials used and their character-

istic values. Silica sand and silica powder were usedas the solid-phase particles to eliminate the influence

of cement hydration. The liquid-phase viscosity was

measured with a B-type rotational viscometer in an

environment of 20C.

4.3.2 Experimental method and measurement items

For the vacuum dewatering experiment in this

section, the same method described in Sect. 4.1was

used. However, the specimen height was set to180 mm and the vacuum dewatering time was set to

20 min. The measurement items were the time

histories of suction pressure, pore water pressure,

and the amount of air.

Specimens were made as follows. For both

specimens of silica sand and silica powder mixed

with water, silica sand and silica powder were

immersed in water for 24 h, after which they were

cast in molds by jigging. After casting, the silica

sand was left to settle for 30 min and the silica

powder for 8 h, respectively. In the next step, thesurplus specimen material and the water that had

collected at the top were removed in order to make

each specimen 180 mm high. For both of the silica

sand and silica powder specimens mixed with oil,

the immersion time was set to 12 h and the settling

time to 60 min. The heights of those specimens

were finalized using the same method.

0

25

50

75


Sampledepth(mm)

t=10 s

t=50 s

t=150 s

t=300 sElapsed time

0

25

50

75

100

125

150

0 0.02 0.04 0.06 0.08 0.1

0 0.02 0.04 0.06 0.08 0.1


Sampledepth(mm)

t=10 s

t=50 s

t=150 s

t=300 s

Elapsed time

(a)

(b)

Fig. 15 Time histories of concrete pore water pressure

distribution (at the end of bleeding). a Specimen height:

105 mm, b Specimen height: 180 mm

Movement of liquid phase

(b)(a)

Coarse aggregate

(solid phase)

Fine aggregate

(solid phase)

Water

(liquid phase)

Air bubbles

(gas phase)

Air bubbles inflate,

increasing the area

of contact with solid

particles

Cement particles

(solid phase)

Fig. 16 The assumed

appearance inside concrete.a Initial state, b Vacuum-

dewatered state



12/19

4.3.3 Experimental results and discussion

Figure17shows time histories of pore water pressure

distribution. At the specimen depth of 0 mm surface

level, the pore water pressure indicates the suction

pressure. Accordingly, the pore water pressure area

(MPa min) of each layer was divided by the suctionpressure area (MPa min) and the average quotient

was defined as the ratio of pore water pressure to

suction pressure (E). This value is shown in (i) of

each figure (a) to (d). The value of E indicates the

degree of influence by capillary tension or viscous

resistance. As the value becomes greater, the capil-

lary tension or viscous resistance becomes less

influential.

The influence of capillary tension is discussed first.

By comparing Fig.17a and b, we see no pore water

pressure distribution in silica sand but find that porewater pressure distribution is generated in silica

powder. The value of E is smaller for the silica

powder than for the silica sand. This indicates that

vacuum dewatering may generate capillary tension

due to inflation of air bubbles caused by negative

pore water pressure. This gradually decreases the

transmission of suction pressure to lower layers and

thus generates pore water pressure distribution. The

decrease of pore water pressure at the specimen depth

of 0 to 10 mm may be due to the resistance of the

dewatering mat and hose.

The influence of viscous resistance is discussed

next. By comparing Fig.17a and c, we see no

depthwise pore water pressure distribution when the

liquid phase is water but pore water pressure

distribution is generated when it is oil. The value ofEis smaller when the liquid phase is water than when

it is oil. Figure 17d shows this tendency more

markedly. Judging from these results, it is considered

likely that vacuum dewatering may have generated

pore water pressure distribution because the forced

movement of the liquid phase caused viscous resis-

tance, gradually reducing the transmission of suction

pressure applied to the lower layers.

5 Estimation of density distribution

and compressive strength distribution

5.1 Estimation flows

Past experimental results [18] indicated that the

vacuum dewatering process is characterized by

movement of only water but almost on solids (cement

and aggregate). As mentioned previously, vacuum

dewatering was confirmed to generate depth-wise

pore water pressure distribution. Considering thesefindings, if the pore water pressure at each layer is

measured, the density distribution in vacuum-dewa-

tered concrete can be calculated.

If the consolidation properties and pore water

pressure distribution properties of various kinds of

concrete are known, the estimation flows shown in

Fig.18 will enable estimation of the qualitative

changes to concrete imposed by vacuum dewatering.

The amount of water removed from each layer at the

end of vacuum dewatering can be calculated by

considering the linear relationship between the log-arithmic value of consolidation pressure (equal to

pore water pressure) and the final amount of

compaction as shown in Fig. 18c.

By applying the equation of consolidation theory

(time history of the amount of compaction shown in

Fig.18d) to each layer, even the time histories of

density distribution and compressive strength distri-

bution in vacuum-dewatered concrete can be esti-

mated (broken-line estimation flow in Fig. 18a).

Table 7 Experimental factor levels

Factor Level

Solid-phase particle size Silica sand (No. 63),

silica powder

Liquid-phase viscosity (Pa s) 0.002 (water), 0.061 (oil)

Table 8 Materials and characteristics

Materials Characteristics

Solid phase

Silica sand

(No. 63)

37.4% remaining on a sieve with a peak

aperture of 106 lma

Silica powder Absolute dry density: 2.60 g/cm3

Specific surface area: 3,300 cm2/g, Absolute

dry density: 2.66 g/cm3

Liquid phase

Water Tap water (viscosity: 0.002 Pa s)

Oil Edible rapeseed oil (viscosity: 0.061 Pa s)

aRefer to JIS G 5901-1974



13/19

(a)

(b)

(ii) Discrete time history


0

30

60

90

120

150

180

0.00 0.02 0.04 0.06 0.08 0.10


Sampledepth(mm)

0.00

0.02

0.04

0.06

0.08

0.10

0 5 10 15 20

Time (min)

Porewater

pressureorsuction

pressure(MPa)

Ratio of pore water pressure to

suction pressure E = 93.5%

Depth 10 mm Depth 50 mm Depth 90 mm

Depth 130 mm Suction pressure

(i) Continuous time history


t=1m in

t=2m in

t=5m in

t=10m in

t=15m in

t=20m in

t=1 min

t=5 min

t=10 min

t=2 min

t=20 min

t=15 min

0.00

0.02

0.04

0.06

0.08

0.10

0 5 10 15 20

Time (min)


pressure(MPa)

0

30

60

90

120

150

180

0.00 0.02 0.04 0.06 0.08 0.10


Sampledepth(mm)



(c)

(ii) Discrete time history(i) Continuous time history


(d)


0.00

0.02

0.04

0.06

0.08

0.10

0 5 10 15 20

Time (min)

Porewaterpressure

orsuction

pressure(MP

a)

0.00

0.02

0.04

0.06

0.08

0.10

0 5 10 15 20

Time (min)

Porewaterpressureorsu

ction

pressure(MPa)

0

30

60

90

120

150

180

0.00 0.02 0.04 0.06 0.08 0.10


Sampledepth(mm)

0

30

60

90

120

150

180

0.00 0.02 0.04 0.06 0.08 0.10


Sampledepth(mm)





Fig. 17 Time histories of

pore water distribution.

a Water ? silica sand

(Air = 1.5%).

b Water ? silica powder

(Air = 0.2%).

c Oil ? silica sand

(Air=

4.9%).d Oil ? silica powder

(Air = 2.5%)



14/19

The estimation flow in Fig.18a is based on the

assumption that the pore water pressure does not

change with the passage of time. If the pressure were

to change with the passage of time, the broken-line

estimation flow in Fig.18a would be equally appli-

cable because it accurately evaluates the time history

of pore water pressure distribution and the amount of

compaction corresponding to pore water pressure.

5.2 Verification of estimation flows using mortar

The estimation flows concerning mortar were verified

using past experimental results [18]. Here, the authors

attempted to estimate the density and compressive

strength distributions that occurred only at the end of

processing where comparison with measured values

was possible. The attempt concerning density distri-

bution was limited to verification by comparison with

component analysis results where the air volume was

not considered.

5.2.1 Pore water pressure distribution

Pore water pressure changes constantly from the start

of processing until its end. For simplification, pore

water pressure was estimated by using the average

(marked s). Figure19shows the average pore water

pressure. In Fig. 19, the authors determined that the

pore water pressure is greater in higher layers and

becomes equal to suction pressure at the surface. To

calculate pore water pressure at an arbitrary depth

z(cm), Eq.4is used. The pore water pressure used forestimation was calculated from four layers as the depth-

wise average of each layer and expressed by Eq. 5.

u u0 eaz 4

uave u0

ae az

h iji= j i 5

where u is the pore water pressure (MPa), uave is the

average pore water pressure by layer (MPa), u0 is

the degree of vacuum (MPa), z is the depth (cm),a is

Pore water pressure distribution

(Constant, irrespective of t)

Start ( Time t = 0 )

Time t = t + t

Division by layer

At the end of processing

Time history

Layer n

Upper layer

Lower layer

Layer n

(d)Relationship between amout of compaction of Layer n and time

Time[Logarithmic scale]

Amountof

compaction

Time t

Value of layer nFinalamountof

compaction

(c)Relationship between final amount of compaction and pore water pressure

Pore water pressure [Logarithmic scale]

(a)Estimation flow

Calculation of dewatering amount

by layer

Calculation of C/W

Density

distribution

Compressive strength

distribution

Calculation of air

volume

(b)Pore Water pressure distribution

Layer n

Lower Layer

Upper layerDegree of vacuum

Depth

Pore water pressure

Fig. 18 Estimation of

density distribution and

compressive strength

distribution (no change in

pore water pressure with the

passage of time)



15/19

the coefficient (a = 0.45), and i, j are the depths of

the upper and lower layers (cm).

5.2.2 Final amount of compaction

Figure20 shows the relationship between the final

amount of compaction and consolidation pressure.

Since the final amount of compaction and the

logarithmic value of consolidation pressure areassumed to be linear within the range of consolida-

tion pressure p from 0.013 to 0.132 MPa [19], their

relationship can be expressed by Eq. 6. With the pore

water pressure uave (MPa) calculated by Eq. 5as the

consolidation pressure p (MPa) in Eq.6, the final

amount of compaction of each layer can be calcu-

lated. In Fig.20, the volume ratio of fine aggregate in

mortar is 50%.

Sf=H b c logp Sf=H= 0 6

where Sf is the final amount of compaction (cm), H isthe layer thickness (cm), p is the consolidation pressure

(MPa),and b, c are thecoefficient (b = 0.063, c = 0.018).

5.2.3 Watercement ratio distribution and density

distribution

To estimate watercement ratio and density, Sf/Hof

each layer is calculated by Eq. 6and the unit quantity

is substituted from the mixing table for Eq. 810.

W=C wUv=cUv 100 7

wUv wUp Sf=H wc 1000

= 1 Sf=H

8

cUv cUp= 1 Sf=H 9

cX

Up Sf=Hwc1000

= 1Sf=H n o

=1000

10

where W/C is the watercement ratio (%), c is thedensity (g/cm3), wUp, wUvare the unit water contents

for no processing and vacuum dewatering (kg/m3),

cUp, cUv are the unit cement contents for no

processing and vacuum dewatering (kg/m3), RUp is

the total sum of non-processed unit contents (kg/m3),

and wc is the water density (= 1.0 g/cm3).

Figures21 and 22 compare the measured and

estimated values of watercement ratio distribution

and density distribution. The measured density distri-

butions are from the values of component analysis

where the air volume is not considered. In both figures,the measured and estimated values match compara-

tively well at the upper, middle, and lower layers.

Therefore, this technique for estimating watercement

ratio distribution and density distribution by vacuum

dewatering seems rather appropriate. In the surface

layer, however, measured and estimated values differ

greatly. This is probably because of the great increase

of water content in the surface layer due to bleeding

(vacuum processing started at the final stage of

0

3

6

9

12

15

18

0 0.02 0.04 0.06 0.08 0.1

Pore water pressure u (MPa)

Sampledepthz(cm)

u = 0.091e ( - 0.45 z)

Measured value (At the end)

Measured value (Average)

Estimated value (Equation(7))

Fig. 19 Pore water pressure distribution

Consolidation pressure p (MPa)

[Logarithmic scale]

Sf/H = 0. 063 +0.018 logp

Experimental value

Estimated value [Equation(6)]

0.08

0.06

0.04

0.02

Finalamountofc

ompactionSf/H

0.10 0.140.040.00

Fig. 20 Relationship between final amount of compaction and

consolidation pressure



16/19

bleeding), and the coefficients b and c in Eq.6 are very

different from those of the inside of concrete, or Sf/His

great at the surface layer. This tendency should be

studied in more detail.

5.2.4 Compressive strength distribution

Compressive strength was assumed to be expressed

by Eq.11, which is the primary equation for cement

water ratio.

Fc d C=W e 11

where Fc is the compressive strength (MPa), C/W is

the cementwater ratio, and d, eare the experimentalcoefficient (d= 45.9, e = 38.8).

Figure23 compares the measured and estimated

values of compressive strength distribution. Accord-

ing to the figure, the measured and estimated values

match comparatively well at every layer. Therefore,

this technique of estimating compressive strength

distribution by vacuum dewatering seems appropri-

ate. The figure also gives the value of the surface

layer for reference.

5.3 Estimated values for varied pore waterpressure

As previously mentioned, the improvement of vac-

uum-dewatered concrete may be susceptible to pore

water pressure distribution. The proposed method

tends to reduce pore water pressure distribution in

lower layers, as shown in Figs. 14and15. The causes

of this tendency may be capillary tension and viscous

resistance, as explained in Sect. 4.3. If the decrease

0

30

60

90

120

150

180

Water-cement ratio (%)

Specimendepth(mm)

No processing

Vacuum

(Measured)

Vacuum

(Estimated)

30 40 50 60 70

Fig. 21 Comparison of measured and estimated watercement

ratio distributions (Mortar)

0

30

60

90

120

150

180

Density (g/cm3)

Specimendepth(mm)

No processing

Vacuum

(Measured)

Vacuum

(Estimated)

2.10 2.15 2.20 2.25 2.30 2.35

Fig. 22 Comparison of measured and estimated density

distributions (Mortar)

0

30

60

90

120

150

180

Compressive strength (MPa)

Specimendepth

(mm)

No processing

Vacuum

(Measured)

Vacuum

(Estimated)

30 40 50 60 70

Fig. 23 Comparison of measured and estimated compressive

strength distributions (Mortar)



17/19

of pore water pressure could be suppressed by some

other method, further improvement could be

expected. The greatest improvement effect may be

available when the pore water pressure distribution

becomes uniform in the depthwise direction (as in the

final state after consolidation). Using the estimation

flows given in Sect. 5.1, this section explains the

relationship between pore water pressure distribution

and expected improvement.

5.3.1 Assumption of pore water pressure distribution

Figure24 shows the assumed pore water pressure

distribution, values being between the experimental

and idealistic ones as follows, in order to examine the

improvement effect from the current state to the

maximum state

(a) Approximated value to the experimental one

(Fig.19, a = 0.45)

(b) Almost average of both curves of (a) and (c)

(a=

0.075)(c) Idealistic value in the case of depthwise unifor-

mity (a = 0.001)

5.3.2 Density distribution and compressive strength

distribution

Under the same conditions used for the verification of

estimation flows in Sect. 5.2, the density distribution

and compressive strength distribution in concrete

were estimated with only the pore water pressure

distribution value as assumed in Fig.24. Figures25

and26show the estimated values. In Fig. 25, the pore

water pressure in the middle layer (9 cm deep) is

almost uniform in (a) 0 MPa, (b) 0.045 MPa, and (c)

0.9 MPa. The density increase rate is greater in (b)

than in (a) but smaller in (c) than in (b). This

tendency is more marked in lower layers. According

to Fig.26, the compressive strength distributionshows a similar tendency. In an area of comparatively

small pore water pressure, therefore, the density and

compressive strength may show significant compar-

ative improvement even when the pore water pressure

increase rate is small. In other words, if the capillary

tension and viscous resistance from the middle to low

layers of concrete can be reduced to suppress even a

minor decrease of pore water pressure, the effect of

vacuum dewatering can be expected even deep in

concrete.

6 Conclusion

This paper reported the applicability of the one-

dimensional consolidation theory to the estimation of

density distribution in vacuum-dewatered concrete.

Based on a conjecture that pore water pressure

distribution in concrete generates concrete density

distribution at vacuum dewatering, the pore water

0

3

6

9

12

15

18

Pore water pressure u(MPa)

Sampledepthz(cm)

(b) a=0.075

(c) a=0.001

(a) a=0.45

0 0.02 0.04 0.06 0.08 0.1

Fig. 24 Assumed pore water distribution

0

30

60

90

120

150

180

Density (g/cm3)

Specimendepth(mm)

No processing

(a)Estimated value (a=0.45)

(b)Estimated value (a=0.075)

(c)Estimated value (a=0.001)

2.10 2.15 2.20 2.25 2.30 2.35

Fig. 25 Estimated density distribution



18/19

pressure was measured as reported here. From the

results, estimation flows for density distribution and

compressive strength distribution by applying the

one-dimensional consolidation theory was proposed.

The acquired data can be summarized as follows:

1. According to results of the press dewatering

experiment using mortar, the relationship

between the amount of compaction and the timematched that estimated by the consolidation

theory comparatively well.

2. The density increases of press-dewatered mortar

and concrete became almost constant in the depth

direction. This tendency matched that estimated

by the consolidation theory.

3. The density distributions in vacuum-dewatered

mortar and concrete were about equal to those of

non-processed mortar and concrete at the bottom

layer and equal to those of 0.1 MPa (equivalent

to atmospheric pressure) press-dewatered speci-mens at the top layer.

4. The technique based on the consolidation theory

was found to enable estimation of density and

compressive strength distributions generated by

vacuum dewatering in mortar. However, the

technique should be further studied in order to

improve the accuracy of estimation concerning

the surface layer where the amount of accumu-

lated cement varies with the filter mat

performance and the consolidation properties

depend on bleeding.

5. The pore water pressure distribution generated

by vacuum dewatering may be attributable to

capillary tension and viscous resistance. This

mechanism was verified by model experiment.

Acknowledgements The authors gratefully acknowledge

financial support provided by the FY2004 Category B of

Scientific Research Grants (research representative:

Hatanaka Shigemitsu) of Japan Society for the Promotion of

Science for supporting our research study. The authors also

thank Mr. Hiroshi Wato (Mie University).

References

1. Billner KP (1952) Applications of vacuum-concrete. ACI J

48:5815912. Kodama T (1962) Characteristics of vacuum-processed

concrete. Cem Sci Concr Technol 16:284289 (in Japanese)

3. Takabayashi T (1968) Vaccum dewatering of concrete.

Riko Tosyo, Japan (in Japanese)

4. Lewis RK, Mattison EN, Smith CJ (1973) The vacuum

dewatering concrete, CSIRO, Report 6

5. Malinowski R, Wenander H (1975) Factors determining

characteristics and composition of vacuum dewatered

concrete. ACI J 72(3):98101

6. Kakizaki M, Wami H, Chen C (1979) Research on floor

slab concreting by vacuum treatment. Annu Rep KAJIMA

Tech Res Inst 27:8184 (in Japanese)

7. Neville AM (1981) Properties of concrete. Pitman pub-

lishing, London8. Hosokawa U, Ozaki S, Sugata N (1989) Durability of

vacuum-processed concrete. Cem Sci Concr Technol

43:210215 (in Japanese)

9. Nakazawa T, Tanigawa K, Kurosaki T (1990) Effect of

vacuum treatment on strength of concrete. Cem Sci Concr

Technol 44:342347 (in Japanese)

10. Yang C, Wang Y, Jiang Z, Gu L (1991) Application of

vacuum dewatering technique on casting concrete floors.

P.Seidler Industriefubboden/Industrial Floors 91, vol 1, pp

185190

11. Ozdemir O (1999) Experimental study on vacuum pro-

cessed concrete for floor constructions. P.Seidler Indus-

triefubboden/Industrial Floors 99, vol 1, pp 253263

12. Hatanaka S, Sakamoto E, Mishima N, Muramatsu A (2008)

Improvement of strength distribution inside slab concrete by

vacuum dewatering method. Mater Struct 41:12351249

13. The Japan Geotechnical Society (1993) Countermeasure

construction method for soft ground. (in Japanese)

14. Mikasa M (1963) Consolidation of soft claynew con-

solidation theory and its application. Kajima Shuppan-kai,

Japan (in Japanese)

15. The Japan Geotechnical Society (2001) The method and

description of soil testing. (in Japanese)

16. Hattori H, Mishima N, Wato H, Hatanaka S (2002)

Experimental study on compaction mechanism of mortar

0

30

60

90

120

150

18030 40 50 60 70

Compressive strength (MPa)

Specimendepth(mm)

No processing

(a)Estimated value (a=0.45)(b)Estimated value (a=0.075)

(c)Estimated value (a=0.001)

Fig. 26 Estimated compressive strength distribution



19/19

based on the consolidation theory. Proceedings of Annual

Meeting of AIJ A-1:471472 (in Japanese)

17. Hattori H, Sakamoto E, Mishima N, Hatanaka S (2004)

Experimental study on pore water pressure distribution in

mortar and concrete during vacuum processing (Part.1

outline of experiment). Proceedings of Annual Meeting of

AIJ A-1: 245246 (in Japanese)

18. Hatanaka S, Hattori H, Sakamoto E, Mishima N (2005)

Study on mechanism of strength distribution in vacuum

processed concrete based on the consolidation theory.

J Struct Constr Eng Archit Ins Jpn 596:18 (in Japanese)

19. Hattori H, Hatanaka S, Mishima N, Wato H (2003) Fun-

damental study on compaction mechanism of mortar based

on the consolidation theory. Proc Jpn Concr Inst 25(1):

881886 (in Japanese)


study on mechanism of strength distribution development

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