コンクリート工学年次論文集，Vol.31，No.1，2009- Technical Paper -EFFECT OF CLOGGING ON PERMEABILITY OF SINGLE AND DOUBLELAYERED POROUS CONCRETE
Sunil PRADHAN*1,Naoki MISHIMA*2 and Shigemitsu HATANAKA*3
ABSTRACTOne of the most prominent issues facing porous concrete is that of clogging where the continuous
voids get choked. This paper is an effort made to first study the clogging and its effect on thecoefficient of permeability of POC, followed by an additional experiment to find a solution for
clogging by creating a double layered POC and further to investigate the effectiveness of the upper
layer made of smaller sized aggregates with smaller value of void ratio, in recovering the permeability
by washing after the voids get clogged by the added silica sand.
Keywords: porous concrete, permeability, clogging, single layer, double layer.
1. INTRODUCTION

Table 1 Factors and testing levels (1st exp.:
single layered POC) [3]Porous concrete (POC) is a technology in whichconcrete has intentionally created continuous voids
inside. These particular continuous voids are designed
to allow the water and air to permeate through them,which in turn will enable POC to exhibit various
environmentally friendly activities. To highlight a few
of them, POC can be used to avoid creation of
imbalance in natural ecosystem by the prevention ofpollution of rivers, lakes and coastal waters, erosion,
flash floods, water table depletion as rainwater rushingacross pavement surfaces picks up everything from oil
and grease spills to deicing salts and chemical fertilizers[1]. It can co-exist with micro-organisms; it can be used
as a media for thermal and moisture-conditioning,plants and grasses can be grown on it to enhance thelook as well as impart greening effect to areas like river
protection works and barren slopes; it can also be
utilized as noise barrier walls in railways andexpressways.
The advantages of POC notwithstanding, there
are a few problems facing POC. Potential areas ofconcern are: effect of cyclic wetting and drying,freezing and thawing, shrinkage cracks, repeatedloading effect, abrasion, wearing and clogging
phenomenon [2].
In this paper, two experimental studies have been
carried out. The First experiment was on single layered
POC samples the purpose of which is to study the
clogging phenomenon and its effect on the coefficientof permeability (KT) [3]. The second experiment was
carried out on single and double layered POC with theaim to study if the coefficient of permeability could berestored by washing, after the occurrence of clogging
phenomenon. The results and discussion in thefollowing sections of the paper will throw light on theoutcome of the experiments.

FactorVoid ratio(%)
Size of Aggregate
Added sand (g)

*1
*2
*3

3

3

Testing Levels
15, 23, 30
G6*1, G7 *20, 60, 120[0, 0.4, 0.8]

2

[Vs/S.A* (cm /cm )]Note: *1 Aggregate Grade G6 (5-13)mm, *2 Grade G7 (2.5-5)mm*3 Vs: added sand volume, S.A: top s urface area (150x 40)mm

Table 2 Factors and testing levels (2nd exp.:
double layered POC)
Fa ctorsSize of Aggrega te
Combination of layers
Added sand (g)[Vs/S.A*4 (cm3/cm2)]

Thickness of top layer (mm)Testing levels
G6*1, G7*2, G8*3

(G6+G7), (G6+G8)
0, 60, 120[0, 0.4, 0.8]
4,8,12

Note:*1 G6 (5-13)mm, *2 G7 (2.5-5)mm, *3 G8 (1.25-2.5)mm* 4 Vs: ad ded sand volume, S.A: top s urface area (150x 40)mm

Table 3 Experimental conditions (2nd exp.:
double layered POC)
W/C ratio (%)
Void ratio of upper layers (%)
Void ratio of under layer (%)
Under layer (50mm)
Size of single/under layer

30%
18%
23%
G6
(50x150x40)mm

Table 4 Mix proportion
Sample

water

G6 *2
G7 *3
*4
G8

544.84167.33
161.5

Unit weight (g/L)
cement Aggregate
1816.13557.55538.578163.791746.111778.87

1

SP*
1.63
0.50
0.48

N ote: *1 SP: Superplasticizer, *2 G6 (5-13)mm, *3 G7 (2.5-5)mm,*4 G8 (1.25-2.5)mm

Graduate Student, Div. of Architecture, Graduate school of Engineering, Mie University, JCI member
Assistant Prof., Div. of Architecture, Graduate School of Engineering, Mie University, Dr. Eng., JCI Member
Professor, Div. of Architecture, Graduate School of Engineering, Mie University, Dr. Eng., JCI Member-1735-

-1736-100

1000

Particle size (μm)

Fig.1 Particle size distribution of G6 silica sandUpper Layer
Under layer(50mm)

Fig.2 A double layered POC sample
Upper layer = 4,8 &12mm thickSingle/underlayer (50mm)

Fig.3 Single/under layer sample (G6)Water

h=50mm
Acrylic board(Photographed face)

Top surface
POC sample

Flow of water

Fig.4 Apparatus used for the experiment [3]
2

G6
G7

1.5

KT (cm/s)

2.2 Second Experiment (Double layered POC)
Table 2 displays the factors and levels and Table
3 shows the experimental conditions for the second
experiment respectively. Table 4 shows the mix
proportion for the three grade aggregate samples G6,
G7 and G8. The procedure of the experiment is thesame as in the first experiment. The only difference
here is the introduction of double layered samples as
opposed to the single layered ones in the firstexperiment. Here, single layered POC samples of grade
G7 and G8 along with double layered samples of
G6+G7 and G6+G8 were prepared for carrying out theclogging versus permeability tests. Fig.2 and Fig.3
represent the double and single layered samples. The
under layer is of G6 aggregate with void ratio of 23%
and the upper layers are of G7 and G8 aggregates withthe void ratio of 18%. In this case, the double layered
samples had three different thicknesses for the upper
layer i.e. 4mm, 8mm and 12mm. The under layer ismaintained of the same size as the sample in the firstexperiment i.e. 50x150x40mm. The void ratio of theupper layer has been intentionally kept lower than thatof the under layer to try and stop as much added sand as
possible on the surface itself and to simultaneously
ensure the easy movement of the sand from the underlayer after it enters from the upper layer. This flowmechanism by virtue of the difference in void ratios andaggregate sizes in the two layers is expected to facilitate
the movement of sand entering in from the upper layer,
till the bottom of the sample.

10

1

0.5
0
0

10

20

30

40

Void ratio (%)
Fig.5 Relationship between
KT and void ratio [3]2○:VR = 15%△:VR = 23%□:VR = 30%
VR: Void Ratio

1.5

KT (cm/s)

2.1 First Experiment (Single layered POC)
Table 1 shows factors and levels of the firstexperiment. This experiment was carried out on single
layered POC samples of size (50x150x40) mm.
Samples with aggregate grades G6 of void ratios 15%,23% and 30% along with G7 of void ratios 23% wereused. Single block of POC was first cast and later cut
into required sizes of the samples. Both the longitudinalfaces of the samples have cut and ground surface tomake them smooth and uniform throughout. This
ensures that the samples fit in the apparatus perfectly sothat water enters only from the top surface of thesample and comes out from the bottom surface. An
indigenous apparatus as shown in Fig.4 was preparedto carry out the experiment. During the experiment, awater head of 50mm as can be seen in figure wasmaintained. In order to calculate the coefficient of
permeability (KT), water permeating through thesample was collected at the outlet for 10 seconds andweighed every time. Normal tap water was first allowed
to pass through the samples. Then 60g each of grade G6
silica sand were added successively, once again taking
the weight of the water collected for 10 seconds after
each addition. The procedure was repeated for all thefour samples. Clogging patterns were observed and its
impact on behavioral change in KT was calculated using
the data obtained from the experiment.

Frequency (%)

2. EXPERIMENTAL METHOD

45
40
35
30
25
20
15
10
5
0

G6
G7

VR=23%For G6
1

0.5

0
0

60

120

Quantity of sand (g)

0

60

120

Quantity of sand (g)(b) Agg. size(a) Void ratio
Fig.6 Effect of addition of sand on KT [3]: Sand

3. RESULTS AND DISCUSSION-1737-

POC

:

Void

(a) Void ratio = 15%(b) Void ratio = 23%(c) Void ratio = 30%
Fig.7 State of sand inside voidsof POC (Agg.size:G6)[3]1.8
1.4

Water only
120g sandAfter w ash

1.2

VR=23%

G6 samples

KT (cm/s)

1.6

1
0.8
0.6
0.4
0.2
0
15

23

30

G6

G7

Aggregate size
void ratio (%)(a) Void ratio(b) Agg. size
Fig.8 Effect of washing on KT [3]1.4

VR of G6=23%
Others = 18%
VR = void ratio

Single layer(4mm upper layer)

1.2(8mm upper layer)(12mm upper layer)

1.0

KT (cm/s)

3.1 First Experiment (Single layered POC)
This experiment demonstrated that thepermeability of POC is inversely proportional to theclogging phenomenon. As the added sand entered thevoids of POC, clogging occurred and consequently
reduced the coefficient of permeability of the sample.
Fig.1 shows particle size distribution of silica sand.
(1) KT of POC before the addition of sandFig.5 illustrates the relationship between void
ratio and KT. It can be seen that the KT increases withascending void ratio. The figure also illustrates that KT
improves with the increase in size of the aggregate. It isclearly visible in the figure that the permeability forgrade G6 sample with 23% void ratio is higher than thatof grade G7 sample with same void ratio.
(2) KT of POC after the addition of sandFig.6 represents how the presence of sand inpermeating water affects the coefficient of permeability(KT) of POC with different void ratios and aggregate
grades. It can be seen that permeability in all the cases
decreases with the increase in amount of sand added.
This goes to show that coefficient of permeability isbound to decline with time in the real situation also as
the amount of foreign materials entering or depositingon the POC pavement increases progressively.
Fig.7 displays the extent to which sand enters POC
samples with different void ratios. The figure brings tolight the fact that greater the value of the void ratio,
greater is the quantity of sand entering the void. In
Fig. 7(a) where the void ratio is 15%, the quantity ofsand that has entered the voids is the minimum andmost of the sand prevails on the surface of the sample
as compared to that with void ratio 30% in Fig.7(c),where the sand has moved till the bottom most part ofthe sample. After the occurrence of clogging
phenomenon, an attempt was made to see how effective
would washing prove in restoring the coefficient of
permeability of POC. For washing the silica sand that
had entered the voids or deposited on the top of thesample, water pressure was applied from the top.
(3) Effect of washing on KT
As illustrated in Fig.8(a), it was observed thatafter washing was carried out, KT decreased for gradeG6 samples with void ratios 15 and 23 %, but a slightincrease though very insignificant, could be noticed incase of the samples with void ratio of 30%. The reason
for this is that the larger amount of voids can facilitate
the movement of sand from top till the bottom of thesample after which it goes out. Whereas, in case oflesser amount of voids, the sand gets stuck somewherein the middle even when water pressure is applied
during washing. In other words, the voids get clogged
due to the inability of the sand entering in, to move till
the bottom of the samples. A comparative study as
shown in Fig.8 (b) was also made between samples ofgrades G6 and G7 with the same void ratio of 23%.In
this case, it was noticed that washing helps to improvethe coefficient of permeability of grade G7 aggregates

:

0.8
0.6
0.4
0.2
0.0
G8

G7

G6

G6+G7 G6+G8

Single and double layered samples
Fig.9 KT under normal condition

-1738-G6 (Single layer)
G7 (Single layer)

1.2

G6+G7 (4mm upper layer)

K T (cm/s)

1

G6+G7 (8mm upper layer)
G6+G7 (12mm upper layer)

0.8
0.6
0.4
0.2
0
0

60

120

Sand added (g)(a) For single and G7 upper layers
1.4

G6 (Single layer)

1.2

G8 (Single layer)
G6+G8 (4mm upper layer)

1

G6+G8 (8mm Upper layer)
G6+G8 (12mm upper layer)

0.8
0.6
0.4
0.2
0
0

60

120

Sand added (g)(b) Single and G8 upper layers
Fig.10 Effect of addition of sand on KT
G6 (Single layer)

100

G7 (Single layer)
G6+G7 (4mm upper layer)

KT (%)

80

G6+G7 (8mm upper layer)
G6+G7 (12mm upper layer)

60

40

20

0
30

60

90

120

150

Sand added (g)(a) Single and G7 upper layers
G6 (Single layer)

100

G8 (Single layer)
G6+G8 (4mm upper layer)

80

G6+G8 (8mm Upper layer)
G6+G8 (12mm upper layer)

KT (%)

3.2 Second Experiment (Double layered POC)
As mentioned earlier, this experiment is a follow
up experiment of the first experiment. Here a double
layered POC is introduced in the effort to prevent theclogging of the voids of POC so that its permeability
can be maintained. For that, the upper layer is designed
to have smaller voids so that majority of the foreignmaterials can be stopped at the upper layer level itself.
(1) KT of different samples on normal condition
Fig.9 represents the coefficient of permeability(KT) of all the samples when only the normal tap wateris allowed to pass through them. It can be seen here thatthough the coefficient of permeability of single layered
G7 and G8 samples are low when their height is 50mm,
it can be increased by reducing their thicknesses. It is
made obvious by the figure where permeability in the
double layered samples with lesser thicknesses of G7
and G8 aggregates in the upper layer are visibly greaterthan their single layers. Furthermore, the figure gives
a clear picture of KT reducing with the increasing
thicknesses of the upper layers.
(2) Effect of addition of sand on KT
Fig.10 illustrates the reduction in the coefficientof permeability (KT) of the samples by the addition of
60g each of silica sand in two batches, respectively.
Fig.10 (a) shows the effect on G6 and G7 single
layered samples along with double layered samples of
G6+G7 with three different thicknesses. Fig.10 (b)shows the effect on G6, G8 and G6+G8 samples. The
upper layers of G7 and G8 grade aggregates are 4mm,
8mm and 12mm thick. It is clearly visible in the figuresthat in all the cases, the KT reduces progressively. The
trend is almost the same even if the values differ.
Though it may seem obvious in mentioning that the KT
decreases more with the increase in amount of the sandadded, but it would be worthwhile as it provides animage of the real situation. This indicates that over thetime, the permeability of a POC pavement is bound to
decline as the deposits of foreign materials on thesurface or in the voids increase inevitably with time.
Therefore, it will be interesting to see what kindof impact washing has on the KT of different samples inconsideration. And should it work, it would provide a
crucial breakthrough to the ever so bothersome
clogging issue in single layered POC.
(3) Reduction of KT by the addition of sandFig.11 portrays the percentage reduction of KT
after the addition of sand. It can be observed that thedecrease is the maximum in case of G8 and G6+G8
samples. It is because most of the sand gets depositedon the surface unlike G6 and G7 samples wherecomparatively more of them enter through the voids inthe process facilitating the permeability to better extent.

1.4

KT (cm/s)

as lesser amount of sand passes through to enter thevoids of the sample. This makes it comparatively easierto recover the permeability by washing away the sanddeposited on the surface of the sample.
An observation of washing becoming effectivewith the increasing value of void ratio or decreasing
size of the aggregate was made through thisexperiment.

60

40

20

0
30

60

90

120

150

Sand added (g)(b) Single and G8 upper layers
Fig.11 Percentage reduction of
KT by addition of sand

1.4
w ater only

1.2

120g sandafter w ash

K T (cm/s)

1

G8 single

0.8
0.6
0.4

G7 single

0.2
0
G6

G7
Single Layer

G8

4mm

8mm
G6+G7

12mm

4mm

8mm

12mm

G6+G8

Fig.12 Effect of washing on KT of single and double layered POC(4) Effect of washing on KT
Fig.12 demonstrates the effect of washing on the
KT of all the samples in consideration. A look at thefigure can give us a fairly good idea of the trend
followed by the KT of different samples with thechanging conditions. It can be seen that there is in fact asignificant improvement in KT for all the samples
except for the grade G6 sample. G8 and G6+G8
samples show noteworthy as well as consistent
recovery of KT on washing.
Noticeable restoration is also visible in case ofG7 and G6+G7 samples. This is because of the factthat with the smaller sizes of the aggregate, the amountof sand entering into the voids is lesser as the voids are
smaller than that of bigger size aggregates even though
the void ratios are the same. Moreover, majority of thesand entering from the smaller voids of G8 aggregate
upper layer should be able to pass through the bigger
voids of G6 aggregate layer underneath. And that isexactly what makes it possible to recover back the KT
to a greater extent as the sand deposited on the surfaceof the samples can be either washed away with thewater pressure or pushed through the under layer.
On the other hand, comparatively more amountof sand can enter through the voids of G7 and G6 +G7
samples because of which the probability of under layer
getting clogged becomes more.
This result is in keeping with the philosophy
behind advocating the double layered POC which is totry and restrain the majority of the foreign materials
from entering into the voids so that they can be washedaway to restore and maintain the permeability. Further
experiments with the combinations of different
thicknesses and void ratios of the layers will have to be
conducted to confirm the mechanism.
(5) Recovery of KT on washingFig.13 represents the recovery percentage ofcoefficient of permeability of double layered POC. This
figure supplements Fig.12 by giving clearer image of
how the KT changes after the washing procedure isapplied for the samples in consideration. A visibly clear
gap in the recovery percentages of the KT exists
between G7 and G8 aggregate double and single layer
samples. As mentioned earlier also, G8 samples display
a much better KT restoration tendency than the G7
samples.

Thickness of samples (mm)Fig.13 Recovery of KT on washing

4. CONCLUSION
This paper includes two experimental studies.
The first experiment was carried out to find the effect of
clogging on the coefficient of permeability (KT) of
single layered POC followed by a second one on double
layered POC. The second experiment is an effort to see
if clogging issue could be addressed with a double
layered POC.
It can be inferred from the first experiment thatclogging reduces the coefficient of permeability of POC.
It reduces more with the increasing amount of added
sand in water passing through it. Washing reduces theKT of G6 aggregate sample with void ratio of 23%. A
slight but insignificant increase in the G6 samples with
30% void ratio was noticed. Interestingly, betterrecovery in KT could be noticed in case of G7 grade
aggregate samples with 23% void ratio.
In the second experiment, it was observed that
G8 and G6+G8 samples provided very good restorationof KT on washing. It was also noticed that G7 andG6+G7 samples performed fairly well in the restoration
efforts. However, G8 samples stood out with muchhigher recovery percentage of up to 70% as opposed toG7 samples which was around 40%. This justifies theenvisioned purpose of the double layered POC which isto restrain the foreign materials from entering the voidsof POC at the surface level of smaller sized aggregateswith smaller sized voids. This way, clogging of voids
can be prevented and at the same time, the debris at thesurface can be washed off to restore the permeability ofporous concrete.
REFERENCE[1] A.Balogh, Pervious concrete pavements, theenvironmentallyfriendly
choice,ConcreteNetwork.com[2] J. Patrick Coughlin, David C. Mays Infiltration andclogging with pervious concrete pavement,
AGU/ASCE hydrology days, Fort Collins, CO,march 2008[3] S.Pradhan, N.Mishima, S.Hatanaka, Fundamental
study on effect of clogging on permeability ofporous concrete, summaries of technical papers ofannual meeting AIJ, pp. 603-604, Sep. 2008

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