THESIS
EFFECTIVENESS OF BACK FLUSHING FOR CLEANING POROUS PAVEMENTS
Submitted by
Nilesh Arjun Shirke
Department of Construction Management
In partial fulfillment of the requirements
For the Degree of Master of Science
Colorado State University
Fort Collins, Colorado
Spring 2007
I
COLORADO STATE UNIVERSITY
December 1, 2006
WE HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER OUR
SUPERVISION BY NILESH ARJUN SHIRKE ENTITLED EFFECTIVENESS OF
BACK FLUSHING FOR CLEANING POROUS PAVEMENTS BE ACCEPTED AS
FULFILLING IN PART REQUIREMENTS FOR THE DEGREE OF MASTER OF
SCIENCE.
Committee on Graduate Work
________________________________________
Dr. Scott Shuler
Adviser/Chair
________________________________________
Dr. Angela Guggemos
________________________________________
Dr. Charles Smith
________________________________________
Dr. Ramchand Oad
(Outside Committee member)
______________________________________
Dr. Larry Grosse
Department Head/Director
II
ABSTRACT OF THESIS
EFFECTIVENESS OF BACK FLUSHING FOR CLEANING POROUS PAVEMENTS
Permeable pavement is an alternative to traditional asphalt and concrete surfacing.
Porous pavement is a permeable pavement surface with a stone reservoir underneath.
Permeable pavement allows storm water to infiltrate into either a storage basin below or
infiltrate directly in to the soil and ultimately recharging the water table, while also
potentially removing pollutants. Thus, pollutants are prevented from entering into the
water system and therefore, porous pavements are considered sustainable construction by
the U. S. Environmental Protection Agency (United States Environmental Protection
Agency, 1999). Porous pavements eventually fill with debris and get clogged, preventing
water infiltration. Thus, the greatest concern with using porous pavement is its
susceptibility to clogging (Field, 1982). Therefore, the overall maintenance goal for
porous pavement is to prevent clogging of the void spaces within the surface. Once
particles and dust gets trapped in the surface pores, it is very difficult to clear it. Many
owners avoid this type of pavement because of the difficulty associated with this
maintenance. Additionally, its failure rate is about 75%, mostly attributed because of
poor design, inadequate construction technique and clogging. Debris gets clogged in the
pores of the pavement making it impermeable. This thesis explores the idea of removing
particles trapped in the pores of the pavement by back-flushing water from the bottom to
III
the top of pavement. This is a true experiment type of research and porous pavement is
constructed in laboratory and experiments and analysis is done on it to know the
efficiency of back-flush and to calculate water pressure required for cleaning it. Four
variables are evaluated which may influence the efficiency of particle removal. These
are: 1) water pressure, 2) type of clogging, 3) size of pores of the porous concrete layer
and 4) number of flushes. The summary of findings suggests that back-flush method of
maintenance on porous pavements works efficiently on the clogging materials evaluated,
is independent of pore size and also effective for low pressure of water to back flush.
Cleaning properties show no fixed trend with combination of different variables. Among
all variables used, pressure is a significant variable in particle removal after back-flush.
Nilesh Arjun Shirke
Construction Management Department
Colorado State University
Fort Collins, CO 80523
Spring 2007
IV
TABLE OF CONTENTS
1
Introduction…………………………………………………………………..2
2
Literature Review…………………………………………………………….6
2.1 Introduction…………………………………………………………..6
2.2 Design of Porous pavements…………………………………………7
2.3 Construction of porous pavements…………………………………...9
2.4 Advantages of Porous Pavements……………………………….......13
2.5 Disadvantages of Porous Pavements………………………………...14
2.6 Maintenance of Porous pavements…………………………………..14
2.7 Costing………………………………………………….....................15
2.8 Conclusion……………………………………………………… …..16
3
Problem Statement…………………………………………………………...17
4
Research Question……………………………………………………………20
5
Methodology………………………………………………………………….22
6
Construction of a Model………………………………………………………24
6.1 Sections of a Model…………………………………………………..25
6.2 Construction of Section 1…………………………………………….28
6.3 Construction of Section 2…………………………………………….30
6.4 Construction of Section 3…………………………………………….32
6.5 Construction of Section 4…………………………………………….34
V
6.6 Construction of Section 5…………………………………………….35
7
Detailed Procedure of Experiments……………………………………………. 37
8
Experiments and Results…………………………………………………………40
8.1 Strength of Porous Concrete…………………………………………41
8.2 Sieve Analysis………………………………………………………..41
8.3 Classifications of clogging material used for clogging by ASTM
D 2487-00 and gradation of soils by ASTM D 2487-00……………..44
8.4 Percentage Removal of Particles After Back-flush…………………..46
8.4 Average Permeabilties and Percentage Removal Recorded on Highly Porous Concrete Sample…………………………………..53
8.5 Average Permeabilties and Percentage Removal –
Recorded on Less Porous Concrete Sample……………………………………..57
9. Analysis of Data……………………………………………………………….....58
10. Result and Conclusion…………………………………………………………...62
11. Future Research………………………………………………………………….63
12. Appendices……………………………………………………………………….64
13. References………………………………………………………………………..68.
VI
ACKNOWLEDGMENTS
I would like to thank my Advisory and Examining Committee members Dr. Scott
Shuler, Dr. Angela Guggemos, Dr. Charles Smith and Dr. Ramchand Oad for their
support and guidance throughout my education, and for their critical review of this
document. Their time, interest, and assistance are greatly appreciated.
I wish to extend special thanks to Dr. Shuler for providing me with the
opportunity to study under his guidance and for his extra effort in helping me to develop
as a M.S. candidate. His continuous instruction, supervision, and enthusiasm helped to
bring this research to fruition. I would like to express my gratitude to Dr. Charles Smith
for his support, great help and letting me to use his facilities for the construction of a
model and experiments. In particular, I extend thanks to Kevin Jones for his support, help
and sincere interest in this research.
I thank Dr. Oad for guiding me regarding my long term career when I was
confused and his valuable time spent while serving on my committee. I would like to
thank Dr. Angela for suggesting this topic of research and her guidance throughout this
research work.
Finally, I extend sincere thanks to my sister and brother in law for their neverending support and encouragement throughout my years of study.
CHAPTER 1
INTRODUCTION
Increased runoff rates from traditionally paved surfaces have increased peak flow
through stream channels causing erosion and stream bank instability along with overland
erosion (Bean, Hunt, Bidelspach, & Smith, 2004; Leopold, Wolman, & Miller, 1964).
Frequently ignored, thermal enrichment is also a critical stressor of aquatic habitats and
ecology downstream of urban areas because of the detrimental impacts of these
contaminants, and increasing concern for the natural environment (James & Langsdorff,
2003).
Additionally, drainage has become more of an issue in site development all over
the world. Porous pavement is an alternative to traditional asphalt or concrete surfaces
which may help alleviate some of these issues. Porous pavement is a permeable pavement
surface with a stone reservoir underneath which is considered as a sustainable
construction process. There are many different types of porous pavements, including:
porous asphalt pavement (PAP), porous concrete pavement (PCP), modular interlocking
concrete bricks with internal drainage cells (MICBID), and modular interlocking concrete
bricks with external drainage cells (MICBEC) (James & Langsdorff, 2003). Porous
pavement allows storm water to infiltrate into either a storage basin below or infiltrate
directly in to the soil through the open cells (Bachtle, 1974) and the underlying
supporting soils. This allows cleaner water to replenish ground water or flow into lakes
2
and streams (Mississippi Concrete Industry Association, 2005). Porous pavement was
constructed for the first time at the Franklin Institute, Philadelphia, Pennsylvania in 1971
(Bachtle, 1974). The void space in porous concrete is in the 15% to 22% range compared
to three to five percent for conventional pavements, offering improved filtration and an
enormous amount of surface area to catch oils and chemical pollutants (Georgia
Stormwater Management Manual, 2002). This practice should only be used on sites with
gentle slopes, permeable soils, and relatively deep water table and bedrock levels. Soils
should be well or moderately well drained. Since, subgrade soils differ in their capacity to
percolate water, the design of porous pavement varies slightly based on soil type. (The
Urban Land Institute, 1992)
Hence, it is a unique and effective pavement that meets growing environmental
demands and is recognized as a best management practice (BMP) for stormwater
management by the U.S. Environmental Protection Agency (The Pervious Company,
2005).
Porous pavement is significantly more expensive than traditional asphalt (CWP,
1998). Porous concrete systems should not be used on slopes greater than 5% with slopes
of no greater than 2% recommended. If low spots do develop in the porous pavement
area, it may be advisable to install drop inlets to divert runoff into the stone reservoir
more quickly (Urban Land Institute, 1981).
There are several methods available for calculating the runoff for design of Porous
Pavements. The rational method is based on direct relationship between rainfall and
runoff. It is expressed by the Rational Formula:
Q=CIA
3
Q = The peak runoff rate (cfs)
C = The runoff coefficient
I = Mean rainfall intensity over a period equal to the time of concentration (in/hr)
A = The area (acres). (Surface System Drainage Design, 2005).
There are four layers in the porous pavement:
1. Porous Concrete Layer/Porous Asphalt Layer – The porous concrete layer consists of
an open-graded concrete mixture usually ranging from depths of 2 to 4 inches depending
on required bearing strength and pavement design requirements. GADOT No. 8 coarse
aggregate (3/8 to No. 16) per ASTM C 33 or No. 89 coarse aggregate (3/8 to No. 50) per
ASTM D 448 is used (Georgia Stormwater Management Manual, 2002). Porous concrete
is designed to attain a compressive strength of 400 to 4000 psi though strengths of 600 to
1500 are more common (National Ready Mix Concrete Association, 2004). The top
porous asphalt course is an open-graded asphalt concrete surface course approximately 24 inches thick. This course consists of porous asphalt concrete containing little sand or
dust, with a pore space of approximately 16% (as compared to 2-3% for conventional
asphalt concrete). (The Urban Land Institute, 1981). Pervious concrete is much more
porous than underlying soils. Typical infiltration rates are 250 to 300 inches per hour
(Permeable Pavement, 2000).
2. Top Filter Layer – Consists of a 0.5 inch diameter crushed stone to a depth of 1 to 2
inches. This layer serves to stabilize the porous asphalt layer and can be combined with
reservoir layer using suitable stone.
3. Reservoir Layer – The reservoir gravel base course consists of washed, bank-run
gravel, 1.5 to 2.5 inches in diameter with a void space of about 40% (GADOT No.3
4
Stone). The depth of this layer depends on the desired storage volume, which is a
function of the soil infiltration rate and void spaces, but typically ranges from two to four
feet. The layer must have a minimum depth of nine inches.
4. Bottom Filter Layer – The surface of the subgrade should be a 6 inch layer of sand
(ASTM C-33 concrete sand or GADOT Fine Aggregate Size No. 10) or a 2 inch thick
layer of 0.5 inch crushed stone, and be completely flat to promote infiltration across the
entire surface. This layer serves to stabilize the reservoir layer, to protect the underlying
soil from compaction, and act as the interface between the reservoir layer and the filter
fabric covering the underlying soil.
Filter Fabric is very important to line the entire trench area, including the sides, with filter
fabric prior to placement of the aggregates in bottom filter layer. The filter fabric serves a
very important function by inhibiting soil from migrating into the reservoir layer and
reducing storage capacity (Georgia Stormwater Management Manual, 2002).
The following diagram shows the typical cross-section of a porous pavement.
Figure 1. Typical Cross-section of Porous Pavements.
5
CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
Urbanization has a detrimental effect on our surface water systems. Increased
runoff rates from traditionally paved surfaces have increased peak flow through stream
channels causing erosion and stream bank instability along with overland erosion (Bean,
Hunt, Bidelspach, & Smith, 2004; Leopold, Wolman, & Miller, 1964).
Thermal
enrichment of storm water is one of most important effects of urbanization in cold-water
fishery areas – it destroys fish and aquatic ecosystems and affects other indicators like
oxygen concentration or pollutant concentration, because of the temperature dependence
of many related processes (James and Verspagen, 1996, James & Langsdorff, 2003). All
over the world the drainage has become more of an issue in the site development. One
solution that is becoming increasingly popular is the use of storm water recharge beds
topped with porous pavements for parking lots (Brown, 2003); in addition, porous
pavement filters some pollutants through the open cells and the underlying supporting
soils and this allows cleaner water to replenish our ground water or flow into our lakes
and streams (Mississippi Concrete Industry Association, 2005). Permeable pavement, or
drainable pavers, use dates from the Roman Empire when large stones with spaces
between the stones were used for drainage (Knapton, 2005).
6
Porous pavement is a special type of permeable pavement which was constructed
for the first time in the Franklin Institute, Philadelphia, Pennsylvania in 1971 (Bachtle,
1974) that allows rain and snowmelt to pass through it, thereby reducing the runoff from
a site and surrounding areas (United States Environmental Protection Agency, 1999).
Unlike traditional concrete or asphalt, pervious pavements typically provide a void
content of 15 to 25 percent, offering improved filtration and an enormous amount of
surface area to catch oils and chemical pollutants (Georgia Concrete and Products
Association, 2005, p. 2). Storm water technology with porous pavements can be built for
the same cost or slightly more than conventional systems (Brown, 2003). It is a unique
and effective pavement that meets growing environmental demands and is recognized as
a Best Management Practice (BMP) for stormwater management by the U.S.
Environmental Protection Agency (The Pervious Company, 2005). It is not difficult to
design and construct a system with appropriate infiltration capacities, but maintaining
infiltration capacity over several years is a challenge due to clogging and maintenance
practices (James & Langsdorff, 2003). This paper emphasizes the appropriate design of
the porous pavements, its maintenance and benefits.
2.2 DESIGN OF POROUS PAVEMENTS
The process of subsurface drainage design focuses on the removal of water from
the pavement structure. Typical components of a subsurface drainage system are: (a) a
base drainage (Stone reservoir), (b) a filter layer, (c) a collector pipe, and (d) an outlet
pipe (UNI-Group U.S.A., 1998) constructed above the existing soil. This practice should
only be used on sites with gentle slopes, permeable soils, and relatively deep water table
and bedrock levels. Soils should be well or moderately well drained (The Urban Land
7
Institute, 1981). Soils need to have permeability between 0.5 and 3.0 inches per hour and
the bottom of the stone reservoir should be completely flat so that infiltrated runoff will
be able to percolate through the entire surface (The Storm water mangers resource center,
1997, p. 1)
Following table shows coefficient of permeability and porosity of various layers
used in the design of the Porous Pavements.
Table 1 Parameters for Porous Pavement Design
Material
Coefficient of permeability Porosity (Voids)
(ft/day)
%
Asphalted concrete surface
200
22
Asphalt treated base
16,000
40
Granular sub-base
23,000
40
Compacted sub-grade
0.004
-
Natural soil
-
25
(Anon, 1987, p. 83)
Porous asphalt pavement is a paved surface and sub-base comprised of asphalt, gravel,
and stone, formed in a manner resulting in a permeable surface and theses various layers,
called ‘courses’, which have the potential for storm water detention (Jian-Shiuh Chen,
Kuei-Yi, & Sian-Yun, 2004).
8
Figure 2. Typical cross-section of porous pavements (The Urban Land Institute,1981, p6)
When out flow capacity of the stone reservoir is less than the infiltration rate, the
infiltrated water will accumulate in the pavement (UNI-Group U.S.A., 1998). Pavement
design is based on the following three separate kinds of flow: (a) percolation through the
pavement to the stone reservoir; (b) lateral flow through the base course to disposal
trenches; and (c) percolation from the disposal trenches into the surrounding soil (Anon,
1987).
An open graded asphalt-concrete mix must be designed for stability and
permeability if it is to be successfully used for a porous pavement (Woelfl, 1981).
Factors that affect permeability, such as grain size distribution and the percent of fines
passing through the #50 sieve, are important considerations and should be carefully
selected by the engineer relative to the retention time and desired amount of storage
capacity (UNI-Group U.S.A, 1998). A geo-synthetic liner and sand layer can be placed
9
below the stone reservoir to prevent preferential flow paths and to maintain a flat bottom
(The Storm water mangers resource center, 1997).
2.3 DESIGN AND CONSTRUCTION OF POROUS PAVEMENT
Many localities determine base thickness with the 1993 Guide for the Design of
Pavement Structures by the American Association of State Highway and Transportation
Officials (AASHTO), but others may have specific local requirements and specifications
(UNI-Group U.S.A, 1998).
2.3.1 An Open Graded Asphalt Concrete Surface Course: The top porous asphalt course
is an open-graded asphalt concrete surface course approximately two to four inches thick
(The Urban Land Institute, 1981. p. 2). Porous asphalt consists of a wearing course of
open graded asphalt concrete laid over a base course of uniformly graded aggregates and
it differs from conventional asphalt concrete chiefly because it contains little dust or sand
(Field, 1982). Asphalt or concrete to be used for porous pavement must have a high
stability and high permeability but when permeability increases, the stability decreases as
aggregate gradation becomes more open graded; still a compromise must be made when
selecting an aggregate specification (Woelfl, 1981, Appendix 2). The void volume, of
porous asphalt, typically is around 16 percent; as compared with two to three percent
void volume of conventional asphalt concrete (Field, 1982, p. 252).
Durability of asphalt concrete increases with increasing asphalt contents and a high
asphalt content is especially important to provide durability for open graded asphalt
concrete since the high porosity of open-graded asphalt concrete promotes rapid
oxidization of the asphalt film surrounding the aggregate particles (Woelfl, 1981).
10
Table 2 Typical* Ranges of Materials Proportions in Pervious Concrete
Cementitious materials
270 to 415 kg/cu m (450 to 700 lb/cu yd)
Aggregate
1190 to 1480 kg/cu m (2000 - 2500 lb/cu yd)
Water-cement ratio (by mass)
0.27 to 0.30***
Aggregate-cement ratio (by mass)
4 to 4.5:1***
Fine-coarse aggregate ratio (by mass)
0 to 1:1****
*** Higher ratios have been used, but reductions in strength and durability may result.
****Addition of fine aggregate will decrease the void content and increase strength.
(Farny, 2004, p. 3).
Following table shows the grading of a porous Asphalt Mix.
Table 3 Standard Porous Asphalt Mixes
US Standard Sieve Size
Percent Passing
1/2 in.
100
3/8 in.
95
#4 (4.75 mm)
35
#8 (2.36 mm)
15
#16 (1.18 mm)
10
#30 (0.600 mm)
2
11
(Adams, 2003, p. 2)
Porous asphalt pavement consists of standard bituminous asphalt in which the
aggregate fines (particles smaller than 600 µm, or the #30 sieve) have been screened and
reduced, allowing water to pass through the asphalt (Adams, 2003)
2.3.2 Filter Layer Course: A one to two-inch thick layer of 0.5-inch stone can be placed
over the reservoir or base course, and manually graded to plan specifications (The Urban
Land Institute, 1981, p. 2). In certain places, such as areas adjacent to curbs, drainage and
utility structures, or over a stabilized base, a geo-textile may be used to prevent migration
of bedding sand (UNI-Group U.S.A., 1998).
2.3.3 Reservoir Course or Base Course or Stone layer: The reservoir course is a base of
1.5 to 3-inch stone bed (The Urban Land Institute, 1981, p. 2) is usually between 18 and
36 inches deep, depending on stormwater storage requirements, frost depth
considerations, and site grading. This depth provides a significant structural base for the
pavement (Adams, 2003, p. 1) Sometimes drainage pipes are used in this course to
increase the drainage through pavement in case of excess rainfall. There are three types
of infiltration pipes; type-1 is a round porous concrete drainage pipe; type-2, a boxshaped porous concrete drainage pipe; type-3, an egg-shaped vinyl chloride pipe with
pores (Wada, Miura, Tada, & Kodaka, 1997). Clean, washed 1.5 to 3-inch aggregate
should be placed in the excavated reservoir in lifts, and lightly compacted with plate
compactors to form the reservoir or base course and the minimum depth of this layer is
usually nine inches (The Urban Land Institute, 1981, p. 2).
12
2.4 ADVANTAGES OF POROUS PAVEMENTS
Porous Pavement allows storm water to percolate into the soil rather than increase
runoff in the storm drains or combined sewer lines (Bachtle, 1974) and provides many
benefits in an urban environment, such as storm-water flow attenuation, aquifer recharge,
and storm-water pollution control (Pratt 1997). By using pervious concrete in a parking
lot of a building, the ambient air temperature will be reduced, requiring less power to
cool the building (The Pervious Company, 2005). Porous pavements reduce the need for
curbs and storm sewer installation or expansion (Field, 1982). Studies in the United
States have shown that the ice and snow cover during midwinter are similar on a porous
asphalt surface as on an impermeable asphalt surface (Ferguson 1994, Magnus, 2000).
But later studies found that a porous pavement is more resistant to freezing than an
impermeable pavement due to higher water content in the underlying soil, which
increased the latent heat in the ground (Magnus, 2000); thus, snow and ice tends to melt
more quickly on porous pavement (Adams, 2003).
The strength and durability of pervious pavement appears to be equal to
traditional pavements. There are several examples of parking lots built more than twenty
years ago with pervious pavement that are still structurally sound and in use (Georgia
Concrete and Products Association, 2005). Additionally, it reduces soil erosion (Field,
1982) and restores natural moisture, benefiting roadside vegetation (Bachtle, 1974). For
large highways on sufficiently permeable subsoil, porous pavement, if applicable, is less
expensive than conventional surfaces since it can greatly reduce the need for storm drains
(Miller, 2005).
13
2.5 DISADVANTAGE OF POROUS PAVEMENTS
The greatest concern with using porous pavement is its susceptibility to clogging
(Field, 1982). Traffic levels and type of usage, as well as sources that may wash
sediment onto the paved surface often reduces infiltration levels (UNI-Group U.S.A.,
1998). Fuel may leak from vehicles and toxic chemicals may leach from asphalt and/or
binder surface and porous pavement systems are not designed to treat these pollutants; so
porous pavement gets clogged if improperly installed or maintained in the heavy traffic
area (United States Environmental Protection Agency, 1999). Use of porous pavement
does create risk of groundwater contamination and has a high failure rate - approximately
75 percent mostly attributed due to poor design, improper construction techniques and
clogging (United States Environmental Protection Agency, 1999).
2.6 MAINTENANCE OF POROUS PAVEMENTS
Estimated costs for an average annual maintenance program of a porous pavement
parking lot are approximately $4,942 per hectare per year ($200 per acre per year)
(United States Environmental Protection Agency, 1999. p. 2). It is not difficult to design
and construct a system with appropriate infiltration capacities, but maintaining infiltration
capacity over several years is a challenge, due to clogging and maintenance practices and
the greatest concern with using porous pavement is its susceptibility to clogging (James,
& Langsdorff, 2003). All porous pavements should be inspected several times in the first
few months after construction, and at least annually thereafter.
Inspections should be conducted after large storms to check for surface ponding that
might indicate local or widespread clogging.
If severe clogging occurs, the entire
structure may have to be replaced (The Urban Land Institute, 1981). Spot clogging of the
14
porous pavement layer can be relieved by drilling half-inch holes through the porous
asphalt layer every few feet and in the cases where clogging occurs in a low spot in the
pavement, it may be advisable to install a drop inlet to route water into the stone reservoir
(The Urban Land Institute, 1981). Clogging can be a result of fine particles accumulating
in void spaces of permeable pavements because smaller particles trap larger particles,
therefore the rate of clogging increases as more fines are trapped (Pratt et al., 1995).
Clogging can be remedied by maintenance, either by vacuum truck, street sweeper or
high pressure washing (Balades et al., 1995; Bean et al., 2004). Periodic maintenance is
critical, and surfaces should be cleaned with a vacuum sweeper at least three times per
year. (Permeable Paving, 2006). Following Table 4 shows the typical maintenance
activities for porous pavement
Table 4. Routine Maintenances for Porous pavement.
(Georgia Stormwater Management Manual, 2002, p 37)
15
Concerns about clogging of pervious pavements can be "designed out", by
reducing erosion and sediment runoff through strategic design and water retaining ground
cover (Georgia Concrete and Products Association, 2005). Runoff from adjoining areas
should be prevented or minimized by grading the surrounding landscape away from the
site, or by installing trenches to collect the runoff (Miller, 2005).
2.7 COSTING
From the results of the 1972 economic analysis, it was found the cost of
conventional pavements with storm sewers to be higher than the cost of an equivalent
porous asphalt installation without storm relief (Field, 1982). But recent studies show
that Porous Pavement is significantly more expensive than traditional asphalt. While
traditional asphalt is approximately 50¢ to $1.00 per square foot, porous pavement can
range from $2 to $3 per square foot, depending on the design (CWP, 1998; The Storm
water mangers resource center, 1997, p. 1). Subtracting the cost of traditional pavement,
this amounts to approximately $45,000 and $100,000 per impervious acre treated, which
would be quite expensive (The Storm water mangers resource center, 1997, p. 1).
2.8 CONCLUSION
Recent studies have found positive results using permeable pavement with respect
to both runoff reduction and water quality improvement.
The use of permeable
pavement, instead of traditional asphalt has been shown to decrease surface runoff and
lower peak discharge significantly (Bean et al., 2004; Pratt 1995). It is a unique and
effective pavement that meets growing environmental demands and is recognized as a
Best Management Practice (BMP) for stormwater management.
In addition to the
relatively strict site constraints for porous pavement, a major limitation to the practice is
16
the poor failure rate it has experienced in the field. Its probability of clogging is very
high and it makes maintenance difficult.
Several studies indicate that, with proper
maintenance, porous pavement can retain its permeability. This sustainable construction
could be a major relief in stormwater and ground water problems if maintenance is
simplified.
17
CHAPTER 3
PROBLEM STATEMENT
It is not difficult to design and construct a system with appropriate infiltration
capacities, but maintaining infiltration capacity over several years is a challenge due to
clogging and maintenance practices (James & Langsdorff, 2003). Porous pavement
requires extensive maintenance.
In addition to owners not being aware of porous
pavement on a site, not performing these maintenance activities is the main reason for
lack of porous pavement utilization.
The susceptibility of a porous pavement to clogging is one of the great concerns
in porous pavement applications (Siew-Ann et al., 2003). The clogging of the void spaces
within the permeable base by foreign particles can severely reduce its drainage capacity.
This will, in turn, reduce the service life of the permeable base layer within the pavement.
(Siew-Ann et al., 2003).
Clogging can be remedied by maintenance, either by vacuum truck, street
sweeper or high pressure washing (Balades et al., 1995; Bean et al., 2004). To maintain
the infiltrative capacity of porous pavements, quarterly vacuum sweeping in conjunction
with jet hosing or jet hosing alone is recommended (Schueler et al., 1992). Therefore, the
installation of porous pavement Best Management Practices (BMP) in regions that lack
the equipment or resources for routine maintenance is not recommended (Pratt et al.,
1995).
18
Permeable paving can be prone to clogging from sand and fine sediments that fill
void spaces and the joints between brick pavers. As a result, it should be used carefully
taking measures to prevent entering sand in the porous pavement area, where frequent
winter sanding is necessary as the sand may clog the surface of the material. A clogged
permeable base layer will suffer a reduction in its drainage and storage capacity because
surface runoff will have difficulty infiltrating into the permeable or reservoir base layer.
Thus, the greatest concern with using porous pavement is its susceptibility to clogging
(Field, 1982). Traffic levels and type of usage, as well as sources that may wash
sediment onto the paved surface often reduces infiltration levels (Uni-Group U.S.A.,
1998). Clogging can be a result of fine particles accumulating in void spaces of
permeable pavements because smaller particles trap larger particles, therefore the rate of
clogging increases as more fines are trapped (Pratt et al., 1995). Soil retention is found to
increase with gradation openness. A narrower particle size distribution implies a more
open base matrix. As gradation openness increases, the amount of soil retained in each
base gradation also increases. This is because more clogging particles are now able to
penetrate deeper into the base and reside in the void spaces. (Siew-Ann et al., 2003). If
severe clogging occurs, the entire structure may have to be replaced (The Urban Land
Institute, 1981). Hence, it has a failure rate of approximately 75 percent (Galli, 1992;
United States Environmental Protection Agency, 1999). Failure has been attributed to
poor design, inadequate construction techniques, soils with low permeability, heavy
vehicular traffic and poor maintenance (Georgia Stormwater Management Manual,
2002).
19
CHAPTER 4
RESEARCH QUESTION
When water is pumped into the stone reservoir of the pavement, it will try to
come out through the clogged porous concrete layer. While coming out from bottom to
the top of the pavement, water should remove the particles and debris trapped in the pores
of the pavement. The result of this flushing should make the pavement permeable again.
The concept to be explored in this research is removing dust particles and other
debris trapped in the pores of the porous concrete pavement by back flushing with water.
This back flushing method is the process of moving water through the pavement from the
bottom to the top with enough pressure to remove the debris particles trapped in the
pores. When water is pumped into the stone reservoir of the pavement, it will try to come
out through the clogged porous concrete layer. While coming out from bottom to the top
of the pavement, water should remove the particles and debris trapped in the pores of the
pavement. The result of this flushing should make the pavement permeable again.
Following figure 2 shows the concept of back flush method
20
Figure 3 shows the Concept of Back Flush Method
21
CHAPTER 5
METHODOLOGY
Main Steps in the experiment
1.
Fabrication of steel frame to mount a model on it.
2.
Construction of a porous concrete layer in 8” diameter pipe which can be
detached form the pavement base layer and stone reservoir.
3.
Installation of model on a steel frame and construction of pavement with stone
layer, filter layer, base layer in an 8” in diameter and 3’ long pipe.
4.
Sieve analysis for clogging materials which is used to clog the pavement by
ASTM c709-98 Splitting Method.
5.
Calculation of initial permeability of the porous concrete layer.
6.
Clogging the porous concrete layer with a clogging material.
7.
Measurement of permeability of clogged porous concrete layer.
8.
Installation of clogged porous concrete layer on the stone reservoir.
9.
Selection of a head and back-flushing water through pavement by opening a valve
connecting to pavement and 10’ long and 8” in diameter pipe which holds a water to
back-flush.
10.
Drainage of water coming out from the top of the porous concrete layer along
with removed clogged material.
22
11.
Detachment the porous concrete layer from the model and checking its
permeability.
12.
Getting detachable porous concrete layer to the model, installation of it on a stone
layer and back flush it for the second time.
13.
Detachment of porous concrete layer from the stone layer and measurement its
permeability after second back-flush.
14.
Repetition of the procedure by randomly selecting variables such as clogging
material, water head, and types of concrete porous pavement (with high or low
permeability).
15.
Analysis of the data for analysis of variances by using SAS software and further
analysis by S-N-K test.
23
CHAPTER 6
CONSTRUCTION OF A MODEL
6.1 SECTIONS OF THE MODEL
6.1.1 Section 1: A two feet long and 8” diameter pipe for a construction of porous
concrete layer inside of it.
6.1.2 Section 2: A three feet long and 8” diameter pipe for construction of filter layers,
stone layer of the pavement in it.
6.1.3 Section 3: A ten feet long and 8” diameter pipe to store water to create head
difference.
6.1.4 Section 4: A 1½” diameter pipe connected to tap water with water flow control
valve in between.
6.1.5 Section 5: A 1½” diameter pipe connected to section 3 and which is used to drain
the water out after back-flushing.
Following figure 4 shows the sketch of a model constructed for the analysis of BackFlush method of maintenance on porous pavements. Figure 5 and figure 6 show the
photograph of actual model constructed in the laboratory.
Figure 5 shows various
sections of the model and figure 6 shows a full view of the model.
24
Figure 4. Diagram of a Model Constructed for the Analysis of Back-Flush on Porous
Pavement.
25
Following figure 5 and figure 6 show the photograph of a model constructed in the
laboratory for analysis of back-flush on porous pavements. Figure 5 shows sections 1 to
5.
Figure 5. A Photograph showing various sections a model constructed for Back-flush
Analysis.
26
Figure 6. A Photograph Showing Full View of a Model of Back-flush Analysis.
27
6.2 CONSTRUCTION OF SECTION 1
Section 1 has a porous concrete layer in it and is constructed with 3/8” aggregates,
No. 2 cement and water. Following Table 5 shows the proportion of materials used to
make porous concrete layer.
Table 5. Proportion of Materials Used to Construct Porous Concrete Layer.
Material
Weight
Cement
600 lbs
Aggregates 3/8”
2900 lbs
Water
242 lbs
The 3/8” aggregates are taken from the stockpile and sampled by ASTM C 702-98
splitting method to make sure it mixed well and represent the sample. Sieve Analysis is
done for 3/8” aggregates as per ASTM C 136-01. Table 6 shows sieve analysis for 3/8”
and Figure 7 shows a graph for sieve analysis. Y-axis is percentage passing and x- axis
shows sieve size.
6.2.1 Sieve Analysis for 3/8” aggregates as per ASTM C 136-01: Standard Method for
Sieve Analysis for 3/8" aggregates as per ASTM C 136-01: Standard Method for Sieve
Analysis.
Sample Type- 3/8" Aggregates
Initial weight (pan) (gm) = 104
Weight of aggregate with pan (gm) = 2373
Weight of aggregate (gm) = 2269
28
Table 6. Sieve Analysis for 3/8” Aggregates
Sieve Size
Individual
Weight
with pan
retained
(gm)
Weight of
Sand
retained
(gm)
141
2290
187
37
2186
83
3/8"
No. 4 (4.75 mm)
No. 8 (2.36 mm)
Cumulative
weight
Cumulative Cumulative
retained
% retained % passing
37
2223
2269
1.63
97.97
100.00
98.37
2.03
0.00
Sieve Analysis for 3/8" Aggregates
120.00
100.00
% Passing
80.00
60.00
40.00
20.00
0.00
3/8"
No. 4 (4.75 mm)
No. 8 (2.36 mm)
Sieve Size
Figure 7. Sieve Analysis of 3/8” aggregates as per ASTM C 136-01: Standard Method for
Sieve Analysis.
A slope of two percent is given to the porous concrete layer to make sure that
after back-flush, clogged particles are taken out into the drainage pipe along with water.
To check the strength of concrete, four cylinders are made of size 4” by 8”, for low and
high porous concrete samples. Curing of concrete is done as per ASTM 192 & C511
29
Storage tank in hydrated lime. Two 1½” holes are drilled exactly above the porous
concrete layer into the side of the section 1. Two 1½”diameter pipes are installed into the
drilled holes to drain the water coming out of the pavement along with removed particles.
Two samples of section 1 are made with different porosity of concrete.
Following Figure 8 is a photo showing front view of section 1 and Figure 9
showing the top view of section 1; where, porous concrete layer can be seen inside of the
section 1.
Figure 8. A Photograph Showing Front view of section 1
30
Figure 9. A Photograph Showing Top view of section 1
6.3 CONSTRUCTION OF SECTION 2
Section 2 has various layers inside of it. There are four layers of the pavement:
Uppermost is detachable porous concrete layer which is 4” thick.
Filter Layer is
underneath of the porous concrete layer and is 2” thick and consist of 0.5” gravel. Third
layer is a stone layer 3’ high and consists of 1½” stones. Stones used in the stone layer
are sampled by ASTM D 73-03 method. Underneath of the stone layer, gravel layer of 1”
thickness is placed with ½” stone as a filter layer. A bell shape funnel is connected at the
bottom of section 2 to apply water pressure gradually and evenly into the pavement.
Section 2 and section 3 are connected by 1½” diameter pipe with flow control valve
attached to it.
31
Following figure 10 shows a photograph of section 2, where filter layer can be seen
which is above the stone layer.
Figure 10. A photograph of Section 3: Filter Layer and Stone Layer (underneath of filter
layer).
32
6.4 CONSTRUCTION OF SECTION 3
Section 3 consists of 10’ long and 8” diameter pipe connected to section 2. Drill
four holes at the distances of 4.115’, 5.23’, and 7.62’ and 10’ from the bottom of a
Section 3 into 8” diameter pipe and install ½” diameter tube in each hole coming all the
way down on the ground. This allows flexibility to choose head of 1.15’, 2.31’, 4.62’ &
7’. These distances are calculated as follows
Head (ft) = Pressure (psi) * Specific gravity.
(Where, specific gravity of water is 2.31).
Therefore, the head of water which is 2.31’ high produces a pressure of
one psi. The analysis of back-flush is done for the pressure of 0.5, 1, 2 and 3 psi. To
create a pressure of 0.5, 1, 2 & 3 psi, the head of 1.15’, 2.31’, 4.62’ & 7’ is required
above the pavement surface. As Section 2 is 3’ long, distances are calculated from bottom
of the section 3 by adding 3’ to get the head required. Excess water is taken out from the
tubes drilled into the Section 3 by plugging all tubes other than the tube at required head.
This gives a freedom of changing head for random sampling.
Following Figure 11 shows a photograph of section 3 which is a 10’ long
and 8” diameter pipe used to create a head difference.
33
Figure 11. A Photograph of Section 3
34
6.5 CONSTRUCTION OF SECTION 4
Section 4 consists of 1½” diameter pipe with a water flow control valve. This
pipe is connected at the tap water at one end and other end is set at the top of section 3 for
filling water in it.
6.6 CONSTRUCTION OF SECTION 5
Section 5 consists of 1½” diameter pipe connected to the Section 3, exactly above
the porous concrete layer to take the water coming out of the porous concrete layer to the
drainage with removed material which is used for clogging.
35
CHAPTER 7
DETAILED PROCEDURE OF EXPERIMENT
1.
Unclamp Section 1 from the model which has porous concrete layer in it.
2.
Get it to the basin to check its permeability. Start pouring two gallons of water
and set the stopwatch at the same time. Let the water drain through the porous concrete.
As soon as water drains completely, stop the stopwatch and record the time.
The
coefficient of permeability was calculated according to the following formula:
k=
QL
Ath
Where,
k = permeability, in/s
Q = quantity of flow, in3
L = length of specimen, in
A = cross-sectional area of specimen, in2
t = interval of time over which flow Q occurs, s
h = difference in hydraulic head across the specimen, in.
(Maupin, 2000).
3.
Select the clogging material according to random sampling to avoid any bias
while choosing variables.
36
4.
Get ten cubic centimeter of clogging material and spray it evenly on the porous
concrete surface.
5.
Pour four gallons of water on the porous concrete layer to make sure that clogging
is properly done.
6.
Measure the permeability of clogged porous concrete layer same as in step 2.
7.
Get the section 1 to the model, place it over section 2 and clamp it making sure
that it is watertight.
8.
Attach section 5 to the section 1.
9.
Choose the head for back-flushing water and plug all tubes at other heads which
are connected to section 3. Keep the tube from the selected head in a bucket (five gallon)
to take excess water out and get an exact head.
10.
Start pouring water in to section 3 by opening water flow control valve of section
5, which connects tap water source and section 3.
11.
Fill the water in the section 3 till it reaches the decided head. Stop pouring into
the section 3 by closing water flow control valve of section 5. Let the excess water flow
in the bucket to get exact head difference.
12.
Open the valve connecting to the section 2 and section 3.
13.
As water flow control valve is opened, water starts flowing through stone layer
with a decided pressure due to head difference. Water comes out of the porous concrete
layer taking clogged material out. The water coming out of the porous concrete layer
with removed material is drained out with section 5.
14.
When water stops coming out of the pavement, the valve connecting section 2 and
section 3 is closed.
37
15.
Detach the section 5 from section 1.
16.
Detach section 1 from section 3 and take it to the basin to check its permeability
after back-flush as per step 2. Make sure that section 1 is plugged with 11/2” stopper.
17.
Record the permeability after back-flush and take section 1 to the model and
attach it to the section 2 for second back-flush.
18.
Attach section 5 to the section 1 and repeat the procedure.
19.
Back-flush water for the second time for the same head and record the
permeability of porous concrete layer by repeating the procedure.
20.
Randomly select head, type of clogging and porosity of concrete layer and repeat
the experiment.
21.
The efficiency of back-flush which is percentage removal of particles are
calculated by following formula:
Percentage removal of clogged particles from the porous concrete after back-flush
=
Permeabili ty after backflush - Permeabili ty of clogged pavement
* 100 %
Initial Permeabili ty - Permeabili ty of clogged pavement
Thus, time is recorded in seconds to drain two gallons of water for different types
of combinations of variables for measuring permeability of concrete. The experiment for
each type of combination is repeated for three times. This time recorded for these three
runs for each combination of variables are listed in Appendix A and B.
38
CHAPTER 8
EXPERIMENTS AND RESULTS
8.1 STRENGTH OF POROUS CONCRETE
Following table 8 shows the results for compressive strength of porous concrete
Table 8. Compressive Strength of Porous Concrete
Compressive Strength-High Compressive Strength -Low
Porous Concrete
Porous Concrete
Cylinder 1
11800 lbs/939 psi
13081 lbs/1041 psi
Cylinder 2
10706 lbs/852 psi
16172 lbs/1287 psi
Average
11253 lbs/ 895.59 psi
14626.5 lbs/1164 psi
8.2 SIEVE ANALYSIS FOR CLOGGING MATERIAL
Sieve Analysis for a clogging material (which is used to clog the porous concrete
layer) is done as per ASTM C 136-01 which is a standard method for sieve analysis for
fine and coarse aggregates. Table 9 and 10 shows the weight and proportion of various
sizes of clogging particles of the clogging sample. Following Figure 12 and 13 shows the
results of sieve analysis for the sand 1& 2 used for clogging.
8.2.1 Sieve Analysis of clogging sand 1 as per ASTM C 136-01: Standard Method for
Sieve Analysis.
39
Initial weight (pan) (gm) = 162.5
Weight of aggregate with pan (gm) = 697.5
Weight of aggregate (gm) = 535
Table 9. Sieve Analysis of sand 1
Sieve Size
3/8"
No. 4
No. 8
No. 16
No. 30
No. 50
No. 100
No. 200
Individual
Weight
Individual
with pan
Weight of Cumulative
retained
Sand
weight
Cumulative
(gm)
(gm)
retained (gm) % retained
0
183.3
237.5
270.9
269.7
256.6
238.3
210.2
0
20.8
75
108.4
107.2
94.1
75.8
47.7
0
20.8
95.8
204.2
311.4
405.5
481.3
529
0.00
3.89
17.91
38.17
58.21
75.79
89.96
98.88
Cumulative
% passing
100.00
96.11
82.09
61.83
41.79
24.21
10.04
1.12
Cummulative % passing
120
100
80
60
Cummulative % passing
40
20
0
Sieve
Size
3/8"
No. 4
(4.75
mm)
No. 8
(2.36
mm)
No. 16
No. 30
No. 50
No. 100
Figure 12. Sieve Analysis for sand 1 as per ASTM C 136-01: Standard Method for Sieve
Analysis.
40
8.2.2 Sieve Analysis for sand 2 as per ASTM C 136-01: Standard Method for Sieve
Analysis.
Initial weight (pan) gm =160
Weight of aggregate with pan (gm) = 1254
Weight of aggregate (gm) = 1094
Table 10. Sieve Analysis of Sand 2 used for clogging
Sieve Size
3/8"
No. 4 (4.75 mm)
No. 8 (2.36 mm)
No. 16
No. 30
No. 50
No. 100
No. 200
Individual
Weight
Weight
with pan of Sand
retained retained
(gm)
(gm)
0
0
240
418
433.5
473.8
321
182.01
Cumulative
weight
retained
0
0
77.5
255.5
271
311.3
158.5
19.51
Cumulative
% retained
0
0
77.5
333
604
915.3
1073.8
1093.31
0.00
0.00
7.08
30.44
55.21
83.67
98.15
99.94
Cumulative
% passing
100.00
100.00
92.92
69.56
44.79
16.33
1.85
0.06
Cummulative % passing
120
100
80
60
Cummulative % passing
40
20
0
Sieve
Size
3/8"
No. 4
(4.75
mm)
No. 8
(2.36
mm)
No. 16
No. 30 No. 50
No.
100
Figure 13. Sieve Analysis of sand 2 as per ASTM C 136-01: Standard Method for Sieve
Analysis.
41
8.3 CLASSIFICATION OF CLOGGING MATERIALS BY ASTM D 2487-00 AND
GRADATION OF SOILS BY ASTM D 2487-00
8.3.3 Classification of sand 1
Cu
= D60/D10 &
Cc = (D30*D30) / (D60*D10)
Where,
Cu = Coefficient of Uniformity
Cc = Coefficient of Curvature
D10, D60 and D30 = Particle size diameters corresponding to 10, 60 and 30 %
respectively, passing on cumulative particle-size distribution.
Graph gives the value for D10 = 0.15, D60 = 1.15 and D30 = 0.4
Hence,
Cu = 8.33
Cc = 1.05
Cu>6 and 3>Cc>1 hence it is a Well Graded Sand (SW).
8.3.4 Classification of sand 2
Cu = D60/D10 &
Cc = (D30*D30) / (D60*D10)
Where,
Cu = Coefficient of Uniformity
Cc = Coefficient of Curvature
42
D10, D60 and D30 = Particle size diameters corresponding to 10, 60 and 30 %
respectively, passing on cumulative particle-size distribution.
A Graph gives the value for D10 = 0.45, D60 = 1.8 and D30 = 0.8
Hence,
Cu = 4
Cc = 0.79
Cu<6 and 1>Cc.
Hence, it is a Poorly Graded Sand (SP).
8.4 PERCENTAGE REMOVAL OF PARTICLES AFTER BACK-FLUSH
Following Table 11 shows the percentage removal of clogged particles after backflushing water through the pavement for various variables like clogging type, porosity of
concrete layer, pressure of back-flush and number of flushes. The efficiency of backflush which is percentage removal of particles are calculated by following formula
Percentage removal of particles =
Permeabili ty after backflush - Permeabili ty of clogged pavement
* 100 %
Initial Permeabili ty - Permeabili ty of clogged pavement
There are two type of porous concrete with different porosity: high and low; two type of
clogging: sand 1 and sand 2. There are two flushes, 1 & 2. These variables are analyzed
for four different types of pressures: 0.5, 1, 2 and 3 psi. For each kind of pressure, three
readings are taken and listed in a table for each variable. The table does not provide
permeability of the porous concrete and list only percentage removal of particles.
43
Table 11 Percentage Removal of Clogged Particles After Back-Flush
Porosity
Clogging
Flush
Pressure
0.5 psi
0.5 psi
0.5 psi
1.0 psi
1.0 psi
1.0 psi
2.0 psi
2.0 psi
2.0 psi
3.0 psi
3.0 psi
3.0 psi
High
Sand 1
%
%
Removal
Removal
for
for
1st Back2nd Backflush
flush
59.24
54.21
77.17
68.06
66.23
52.53
87.01
80.10
69.75
52.44
87.87
78.53
65.43
72.25
85.12
79.30
78.26
63.19
94.92
83.21
65.25
72.43
88.13
80.35
Low
Sand 2
% Removal
for
1st Backflush
%
Removal
for
2nd Backflush
54.30
91.08
73.51
56.00
95.15
59.62
60.59
94.68
56.78
81.85
61.30
84.63
71.20
59.25
49.41
49.41
83.67
59.27
58.67
75.28
43.81
74.00
69.80
74.23
44
Sand 1
%
%
Removal
Removal
for
for
1st Back2nd Backflush
flush
52.76
70.21
74.48
72.61
55.11
39.61
75.01
82.96
54.93
65.71
73.66
89.74
53.53
74.08
68.25
78.34
45.49
66.79
70.28
90.21
81.44
82.42
81.17
94.61
Sand 2
%
%
Removal
Removal
for
for
1st Back2nd Backflush
flush
64.30
48.26
67.41
45.08
82.51
64.08
43.80
81.08
87.39
79.56
95.99
94.53
72.78
49.98
80.49
46.74
83.47
77.19
58.91
69.53
80.48
84.08
81.75
90.10
Following Figures 15, 16, 17 and 18 show chart of percentage removal of
particles after back-flush. X-axis shows the percentage removal ranging from 0-100
percent and y-axis shows the water pressures used for back-flush. There are four types of
pressures used- 0.5, 1, 2, and 3 psi. Figure 15 is a chart showing percentage removal of
particles after back-flush for a pressure of 0.5 psi which is basically, average of three
readings taken. Similarly, Figures 15, 16 and 17 show chart of percentage removal of
particles after back-flush for a pressure of 1 psi, 2 psi and 3 psi respectively. High and
Low signifies the porous concrete sample with high permeability and low permeability.
45
100.0
90.00
80.00
70.00
High- % Removal for 1st Back-flush
(Clogging-sand 1)
High- % Removal for 2nd Back-flush
(Clogging- sand 1)
High- % Removal for 1st Back-flush
(Clogging-Sand 2)
High- % Removal for 2nd Back-flush
(Clogging-Sand 2)
Low- % Removal for 1st Back-flush
(Clogging- sand 1)
Low- % Removal for 2nd Back-flush
(Clogging- sand 1)
Low- % Removal for 1st Back-flush
(Clogging-Sand 2)
Low- % Removal for 2nd Back-flush
(Clogging-Sand 2)
60.00
50.00
40.00
%REMOVAL
30.00
20.00
10.00
0.00
PRESSURE- 0.5 PSI
PRESSURE
Figure 15. Percentage Removal of Clogged Particles for Pressure 0.5 psi (Average of three readings)
46
100.00
90.00
80.00
70.00
High- % Removal for 1st Back-flush
(Clogging-sand 1)
High- % Removal for 2nd Back-flush
(Clogging- sand 1)
High- % Removal for 1st Back-flush
(Clogging-Sand 2)
High- % Removal for 2nd Back-flush
(Clogging-Sand 2)
Low- % Removal for 1st Back-flush
(Clogging- sand 1)
Low- % Removal for 2nd Back-flush
(Clogging- sand 1)
Low- % Removal for 1st Back-flush
(Clogging-Sand 2)
Low- % Removal for 2nd Back-flush
(Clogging-Sand 2)
60.00
50.00
40.00
% REMOVAL
30.00
20.00
10.00
0.00
PRESSURE- 1.0 PSI
PRESSURE
Figure 16. Percentage Removal of Clogged Particles for Pressure 1 psi (Average of three readings)
47
100.0
0.00
90.00
80.00
High- % Removal for 1st Back-flush
(Clogging-sand 1)
High- % Removal for 2nd Back-flush
(Clogging- sand 1)
High- % Removal for 1st Back-flush
(Clogging-Sand 2)
High- % Removal for 2nd Back-flush
(Clogging-Sand 2)
Low- % Removal for 1st Back-flush
(Clogging- sand 1)
Low- % Removal for 2nd Back-flush
(Clogging- sand 1)
Low- % Removal for 1st Back-flush
(Clogging-Sand 2)
Low- % Removal for 2nd Back-flush
(Clogging-Sand 2)
70.00
60.00
50.00
% REMOVAL
40.00
30.00
20.00
10.00
0.00
PRESSURE- 2.0 PSI
PRESSURE
Figure 17. Percentage Removal of Clogged Particles for Pressure 2 psi (Average of three readings)
48
100.00
90.00
80.00
High- % Removal for 1st Back-flush
(Clogging- sand 1)
High- % Removal for 2nd Back-flush
(Clogging- sand 1)
High- % Removal for 1st Back-flush
(Clogging-Sand 2)
High- % Removal for 2nd Back-flush
(Clogging-Sand 2)
Low- % Removal for 1st Back-flush
(Clogging- sand 1)
Low- % Removal for 2nd Back-flush
(Clogging- sand 1)
Low- % Removal for 1st Back-flush
(Clogging-Sand 2)
Low- % Removal for 2nd Back-flush
(Clogging-Sand 2)
70.00
60.00
50.00
% REMOVAL
40.00
30.00
20.00
10.00
0.00
PRESSURE-3.0 PSI
PRESSURE
Figure 18. Percentage Removal of Clogged Particles for Pressure 3 psi (Average of three readings)
49
8.4 AVERAGE PERMEABILTIES AND PERCENTAGE REMOVAL RECORDED ON
HIGHLY POROUS CONCRETE SAMPLE
Following Table 12 shows the average permeability and percentage removal
recorded on highly porous concrete layer. There are three readings taken for each kind of
pressure. These readings are listed in appendix C and D. The numbers in the each cell of
the following table show the average of three readings. Where, k-Initial is permeability
recorded before clogging porous concrete layer, k-clogging is permeability recorded after
clogging porous concrete layer and k-clean indicates the permeability after back-flush.
There are two clogging types, sand 1 and sand 2 which are shown in second row of the
table. The efficiency of back-flush is a percentage removal of particles and calculated by
following formula,
Percentage removal of particles 
k (Backflush ) - k (Clogged)
* 100 %
k (Initial) - k (Clogged)
Where,
k = Permeability (in/hr)
k (Initial) = Initial permeability of porous concrete layer
k (Clogging) = Permeability of porous concrete layer after Clogging
k (Back-flush) = Permeability of porous concrete layer after back-flush
50
Table 12. Average Permeability Recorded on Highly Porous Concrete Sample
Porosity of
concrete
High
Cloggging
Material
Permeabilty
(k)
Sand 1
k-Initial
kClogging
before
clogging
after
clogging
kClean
1st
Backflush
in/hr
in/hr
in/hr
k-Clean
2nd
Backflush
in/hr
Sand 2
%
Removal
for
1st
Backflush
%
Removal
for
2nd
Backflush
%
%
1st Backflush
k-Clean
for
2nd
Backflush
%
Removal
for
1st
Backflush
%
Removal
for
2nd
Backflush
in/hr
in/hr
%
%
k-Initial
kClogging
k-Clean
before
clogging
after
clogging
in/hr
in/hr
Pressure
Level
0.5 psi
947.97
404.09
746.19
805.02
63.54
74.27
947.97
522.38
842.38
796.53
72.96
59.96
1.0 psi
956.78
460.99
771.60
827.64
62.27
73.58
956.78
553.71
834.35
809.55
70.25
64.12
2.0 psi
951.73
489.74
855.45
866.12
78.96
81.13
951.73
549.76
825.63
781.15
70.68
59.25
3.0 psi
970.97
444.35
814.07
861.58
72.95
80.31
970.97
520.65
864.38
848.29
75.93
72.68
51
Following Figure 19 is a chart for the data listed in above Table 12. Axis-x shows the
different pressures used for back-flush water through pavement and axis-y shows the
percentage particle removal after back-flush.
. There are two flushes, 1 & 2 are used to clean the pavement. Table 12 provides
the data for this chart which is average of permeability for different readings recorded on
high porous pavement.
52
100.0
90.00
80.00
70.00
% Removal for 1st Back-flush
(Clogging-Sand 1)
60.00
flush
% Removal for 2nd Back1flush
)
(Clogging- Sand 1)
Back
% Removal
for 1st Back-flush
1(Clogging-Sand
)
2)
flush
% Removal for 2nd Back2flush
)
(Clogging-Sand 2)
Back2)
50.00
% REMOVAL
40.00
R
EMOVAL
30.00
20.00
10.00
0.00
PRESSURE PRESSURE- 0.5
PRESSURE- 1.0
PRESSURE- 2.0
PSI
PSI
PSI
PRESSURE
0.5
1.0
I
2.0
I
PRESSURE
PRESSURE- 3.0
PSI
3.0
I
I
Figure 19. Percentage Removal of Clogged Particles 53
for High Porous Concrete (for average of all readings taken)
E
8.5 AVERAGE PERMEABILTIES AND PERCENTAGE REMOVAL RECORDED ON
LESS POROUS CONCRETE SAMPLE
Following Table 13 shows the average permeability recorded on less porous
concrete layer.
There are three readings taken for each kind of pressure.
permeabilities in the table show the average of three readings.
54
The
Table 13. Average Permeability Recorded on Less Porous Concrete Sample
Porosity of
concrete
Cloggging
Material
Permeabilty
(k)
Low
Sand 1
k-Initial
kClogging
after
clogging
k-Clean
1st
Backflush
k-Clean
for
2nd
Backflush
before
clogging
in/hr
in/hr
in/hr
in/hr
Sand 2
%
Removal
for
1st
Backflush
%
Removal
for
2nd
Backflush
k-Initial
kClogging
after
clogging
k-Clean
1st
Backflush
k-Clean
for
2nd
Backflush
before
clogging
in/hr
in/hr
in/hr
in/hr
%
Removal
for
1st
Backflush
%
Removal
for
2nd
Backflush
Pressure
0.5 psi
470.27
337.67
427.80
438.78
69.30
76.92
470.27
328.68
432.46
437.75
72.59
76.38
1.0 psi
478.39
341.04
427.32
443.14
62.16
72.11
478.39
339.11
434.34
444.13
68.38
75.55
2.0 psi
476.65
301.62
450.24
454.44
84.81
87.36
476.65
330.70
456.08
455.62
85.74
86.19
3.0 psi
487.62
313.84
465.74
474.02
87.02
92.05
487.62
326.05
467.73
468.86
87.63
88.61
55
Following Figure 20 shows graph for percentage particle removal for less porous
concrete sample taken for average of readings. Axis-x shows the different pressures used
for back-flush water through pavement and Axis-y shows the percentage particle removal
after back-flush.
56
Restoration of Permeability of Less Porous Concrete after Back-flush
100.00
90.00
80.00
70.00
% Removal for 1st Backflush (Clogging-Sand 1)
% Removal for 2nd Backflush (Clogging-Sand 1)
% Removal for 1st BackFlush (Clogging-Sand 2)
% Removal for 2nd Backflush (Clogging-Sand 2)
60.00
50.00
% REMOVAL
40.00
30.00
20.00
10.00
0.00
PRESSURE
PRESSURE0.5 PSI
PRESSURE1.0 PSI
PRESSURE2.0 PSI
PRESSURE3.0 PSI
PRESSURE
Figure 20. Percentage Removal of Clogged Particles57
for Less Porous Concrete (for average of all readings taken)
CHAPTER 9
ANALYSIS OF DATA
9.1 A full factorial analysis of variance (ANOVA) was conducted to determine whether
the independent variables or their interactions had a statistically significant effect at
=0.05 on the cleaning ability of the back flush process. Independent variables used in
the experiment were: water pressure, clogging material, porosity of the concrete layer and
number of flushes. A summary of the results is shown below: Presure levels H, M, L &
VL signifies the pressure of 0.5, 1, 2 & 3 psi
The SAS System
The GLM Procedure
Class Level Information
Class
Levels
Values
Pressure
4
H L M VL
Porosity
2
High Low
Clogging
2 Sand 1 Sand 2
Flush
2 Q1 Q2
Number of Observations Read
Number of Observations Used
96
96
The SAS System
The GLM Procedure
Dependent Variable: Premove
Sum of
Squares
Mean Square
Source
DF
Model
31
6583.34033
212.36582
Error
64
12719.02260
198.73473
Corrected Total
95
19302.36293
58
F Value
Pr > F
1.07
0.4013
R-Square
Coeff Var
Root MSE
0.341064
19.83979
14.09733
DF
Type I SS
Source
Pressure
Porosity
Clogging
Flush
Pressure*Porosity
Pressure*Clogging
Pressure*Flush
Porosity*Clogging
Porosity*Flush
Clogging*Flush
Pressu*Porosi*Cloggi
Pressu*Porosit*Flush
Porosi*Cloggin*Flush
Pressu*Cloggin*Flush
Pres*Poro*Clog*Flush
3
1
1
1
3
3
3
1
1
1
3
3
1
3
3
Source
DF
Pressure
Porosity
Clogging
Flush
Pressure*Porosity
Pressure*Clogging
Pressure*Flush
Porosity*Clogging
Porosity*Flush
Clogging*Flush
Pressu*Porosi*Cloggi
Pressu*Porosit*Flush
Porosi*Cloggin*Flush
3
1
1
1
3
3
3
1
1
1
3
3
1
Premove Mean
71.05583
Mean Square
3325.215458
6.211837
79.570417
93.102204
652.060537
668.936625
71.115388
265.268504
121.770150
674.690104
71.967954
54.522292
198.030150
22.284204
278.594508
Type III SS
F Value
1108.405153
6.211837
79.570417
93.102204
217.353512
222.978875
23.705129
265.268504
121.770150
674.690104
23.989318
18.174097
198.030150
7.428068
92.864836
Mean Square
Pr > F
5.58 0.0018
0.03 0.8602
0.40 0.5291
0.47 0.4962
1.09 0.3583
1.12 0.3468
0.12 0.9485
1.33 0.2523
0.61 0.4366
3.39 0.0700
0.12 0.9476
0.09 0.9645
1.00 0.3219
0.04 0.9902
0.47 0.7061
F Value
3325.215458 1108.405153
6.211838
6.211838
79.570417
79.570417
93.102204
93.102204
652.060537
217.353512
668.936625
222.978875
71.115388
23.705129
265.268504
265.268504
121.770150
121.770150
674.690104
674.690104
71.967954
23.989318
54.522292
18.174097
198.030150 198.030150
5.58
0.03
0.40
0.47
1.09
1.12
0.12
1.33
0.61
3.39
0.12
0.09
1.00
Pr > F
0.0018
0.8602
0.5291
0.4962
0.3583
0.3468
0.9485
0.2523
0.4366
0.0700
0.9476
0.9645
0.3219
The SAS System
The GLM Procedure
Dependent Variable: Premove
Source
Pressu*Cloggin*Flush
Pres*Poro*Clog*Flush
DF
Type III SS
Mean Square
F Value
Pr > F
3
3
22.284204
278.594508
7.428068
92.864836
0.04
0.47
0.9902
0.7061
59
Student-Newman-Keuls Test for Premove
NOTE: This test controls the Type I experimentwise error rate under the complete null hypothesis
But, not under partial null hypotheses.
Alpha
Error Degrees of Freedom
Error Mean Square
Number of Means
Critical Range
0.05
64
198.7347
2
8.1301265
3
9.7646019
4
10.734814
Means with the same letter are not significantly different.
SNK Grouping
A
A
B A
B
B
B
B
Mean
79.953
N
24
Pressure
H
72.753
24
M
66.196
24
VL
65.321
24
L
The analysis indicates that the pressure of water used for back-flush is the most
influencing factor among all the variables used in this experiment. ANOVA indicates an
F-statistic for pressure as 5.58 and its power (P-value) is 0.0018 which is less than α=
0.05 making pressure highly significant. This analysis clearly shows that pressure is a
significant variable but it does not show which pressure (0.5, 1, 2 and 3 psi) in particular
is significant. Therefore, further analysis is done by Student-Newman-Keuls (S-N-K)
test for comparing all four pressure levels. According to this analysis 3 psi pressure level
is significantly different from 0.5, 1 and 2 psi pressure levels. Also, 0.5, 1, and 2 psi
pressure levels are not significantly different from each other. Two way interactions
between variables show that the combination of Type of clogging material*Number of
Flushes is significant among all six combinations of variables. It is clearly represented by
the P-value (0.07) for interaction model (clogging*flush), which slightly more than α
(0.05). Three way interactions give the significance of interaction between three
60
variables.
Analysis shows that there is no significant interaction among four
combinations of three variables in each group. Four way interaction also doesn’t show
the significance when four variables are considered simultaneously.
Increase in number of flushes does not give significantly increassed removal of
particles from the porous concrete layer. Second flush is not very significant in cleaning
as most of the particles trapped in the porous concrete layer are taken out in the first
flush. The F-value in SAS analysis for flush is 0.47 and P-value is 0.4962 which is
greater than α (0.05). This indicates that less water would be used to clean the pavement
as there is no need of many flushes to clean the pavement efficiently
Data does not show any fixed pattern in removing clogged particles trapped in the
porous concrete layer with combination of variables but as it is initially assumed that all
variables are significant in particle removal is proved incorrect. Pressure of water used
for back-flush is the only significant variable in cleaning. Efficiency of particle removal
doesn’t vary much with different pressure levels though water pressure of 3 psi shows
significance among all pressure levels for less porous concrete than high porous concrete.
61
CHAPTER 11
RESULTS AND CONCLUSION
Back-flushing could be a big relief for porous pavement owners as other
maintenance methods like sweeping, vacuuming, pressure washing etc which makes
maintenance difficult. The maintenance of the porous pavements would be simplified to
greater extent. Back-flushing water monthly or quarterly depending upon susceptibility
for clogging would increase the life of porous pavement. This would lead to increased
use of porous pavements resulting greater relief in storm water and ground water
problems as water is directly infiltrated into the soil. This research indicates that high
pressure (3 psi) is significant for particle removal but efficiency of cleaning for 0.5
pressures is also fair. Additionally, more than one flush is not needed in cleaning the
porous pavement which may result into water savings.
62
CHAPTER 12
FUTURE RESEARCH
This is a very basic research done for analysis of back-flushing water through
porous concrete pavements for cleaning and maintenance. For the first time the idea of
cleaning porous pavement with back-flushing water bottom of the pavement to the top is
analyzed. This is a laboratory study and needs more research to implement it in the field.
A very low pressure of water (0.5 psi) also cleans pavement effectively, although as
pressure increases, cleaning ability also increases. It would be possible to create such a
small pressure in porous pavement on the field. Sometimes, drainage pipes are used to
drain excess water through the porous pavement in the region of heavy rain. These
drainage pipes can be used to pump the water into the pavement for back-flushing. Also,
analysis of back-flushing can be done on porous asphalt by using different clogging
materials like soils and sands of different properties and gradation etc.
63
Appendix A
Table 14 Time Recorded on High Porous Concrete Layer to Drain Two Gallons of Water
Porosity
of
concrete
Cloggging
Material
Time (t)
Pressure
0.5 psi
0.5 psi
0.5 psi
1.0 psi
1.0 psi
1.0 psi
2.0 psi
2.0 psi
2.0 psi
3.0 psi
3.0 psi
3.0 psi
High
t-Initial
Before
clogging
(Second)
15.2
15.2
15.2
15.06
15.06
15.06
15.14
15.14
15.14
14.84
14.84
14.84
Sand 1
t-Clogging t-Clean
After
1st Backclogging
flush
(Second)
(Second)
38.71
37.59
31.55
36.4
29.17
29.22
33.17
25.97
30.02
47
27.56
28.61
20.2
20.9
17.24
18.53
18
19.56
16.29
16.51
17.81
22
15.72
16.55
t-Clean for
2nd Backflush
(Second)
19.24
18.21
16.47
17.14
16.83
18.33
15.57
16.28
18.29
18.29
15.7
16.39
64
t-Initial
Before
clogging
(Second)
15.2
15.2
15.2
15.06
15.06
15.06
15.14
15.14
15.14
14.84
14.84
14.84
Sand 2
t-Clogging
t-Clean
After
1st Backclogging
flush
(Second)
(Second)
22
28.5
35.44
27.78
25.21
25.24
26.3
23.42
29.64
27.94
26.47
28.71
17.7
15.86
17.91
18.86
15.36
17.99
18.18
15.43
19.2
16.22
17.88
16.03
t-Clean for
2nd Backflush
(Second)
17.12
17.56
19.81
19.6
16.12
18.02
18.36
16.59
20.88
16.9
17.11
16.95
Appendix B
Table 13 Time Recorded on Low Porous Concrete Layer to Drain Two Gallons of Water
Porosity of
concrete
Cloggging
Material
Time (t)
Pressure
0.5 psi
0.5 psi
0.5 psi
1.0 psi
1.0 psi
1.0 psi
2.0 psi
2.0 psi
2.0 psi
3.0 psi
3.0 psi
3.0 psi
Low
t-Initial
Before
clogging
(Second)
30.64
30.64
30.64
30.12
30.12
30.12
30.23
30.23
30.23
29.55
29.55
29.55
Sand 1
t-Clogging t-Clean
After
1st Backclogging
flush
(Second)
(Second)
44
39.59
44.81
39.4
48.17
40.22
50.33
48.01
45.25
43.98
45.36
48.64
33.51
31.98
35.77
32.42
34.26
34.56
32.62
30.99
32.45
31.52
31.02
30.3
t-Clean for
2nd Backflush
(Second)
33.44
31.8
33.33
32.14
31.57
33.93
32.61
30.72
31.85
30.01
31.28
29.94
65
t-Initial
Before
clogging
(Second)
30.64
30.64
30.64
30.12
30.12
30.12
30.23
30.23
30.23
29.55
29.55
29.55
Sand 2
t-Clogging
t-Clean
After
1st Backclogging
flush
(Second)
(Second)
48.43
39.91
44
44.28
41.22
42.09
39.98
48.48
43.08
38.65
48
47.22
33.56
33.18
33.22
33.04
31.98
34.61
31.82
32.26
30.74
30.48
31.26
30.69
t-Clean for
2nd Backflush
(Second)
33.55
33.09
32.14
32.87
31.82
32.66
31.38
32.92
30.66
30.27
31.41
30.54
Appendix C
Table 14 Permeability Recorded on High Porous Concrete (For all set of readings)
Porosity of
concrete
Cloggging
Material
Permeabilty
(k)
Pressure0.5 psi
Pressure0.5 psi
Pressure0.5 psi
Pressure1.0 psi
Pressure1.0 psi
Pressure1.0 psi
Pressure2.0 psi
Pressure2.0 psi
Pressure2.0 psi
Pressure3.0 psi
Pressure3.0 psi
Pressure3.0 psi
High
Sand 1
k-Initial
kClogging
Before
clogging
in/hr
After
clogging
in/hr
kClean
1st
Backflush
in/hr
Sand 2
kClean
for
2nd
Backflush
in/hr
%
Removal
for
1st
Backflush
%
%
Removal
for
2nd
Backflush
%
1st Backflush
in/hr
kClean
for
2nd
Backflush
in/hr
%
Removal
for
1st
Backflush
%
%
Removal
for
2nd
Backflush
%
k-Initial
kClogging
k-Clean
Before
clogging
in/hr
After
clogging
in/hr
947.97
372.23
713.33
748.92
59.24
65.43
947.97
654.96
814.08
841.66
54.30
71.20
947.97
383.32
689.43
791.28
54.21
72.25
947.97
505.59
908.52
820.57
91.08
59.25
947.97
456.71
835.80
874.87
77.17
85.12
947.97
406.58
804.53
727.37
73.51
49.41
956.78
395.86
777.61
840.68
68.06
79.30
956.78
518.69
764.01
735.16
56.00
49.41
956.78
493.97
800.51
856.16
66.23
78.26
956.78
571.57
938.10
893.87
95.15
83.67
956.78
493.13
736.67
786.10
52.53
63.19
956.78
570.89
800.95
799.62
59.62
59.27
951.73
434.40
884.54
925.45
87.01
94.92
951.73
547.88
792.58
784.81
60.59
58.67
951.73
554.84
872.75
885.08
80.10
83.21
951.73
615.25
933.84
868.55
94.68
75.28
951.73
479.99
809.05
787.82
69.75
65.25
951.73
486.14
750.48
690.09
56.78
43.81
970.97
306.58
654.96
787.82
52.44
72.43
970.97
515.72
888.36
852.61
81.85
74.00
970.97
522.83
916.61
917.78
87.87
88.13
970.97
544.36
805.88
842.15
61.30
69.80
970.97
503.64
870.65
879.14
78.53
80.35
970.97
501.89
898.89
850.10
84.63
74.23
66
Appendix D
Table 15 Permeability Recorded on Low Porous Concrete (For all set of readings)
Porosity
Cloggging
Material
Permeabilty
(k)
Pressure0.5 psi
Pressure0.5 psi
Pressure0.5 psi
Pressure1.0 psi
Pressure1.0 psi
Pressure1.0 psi
Pressure2.0 psi
Pressure2.0 psi
Pressure2.0 psi
Pressure3.0 psi
Pressure3.0 psi
Pressure3.0 psi
Low
Sand 1
Sand 2
k-Clean
for
2nd
Backflush
in/hr
%
Removal
for
1st
Backflush
%
%
Removal
for
2nd
Backflush
%
k-Initial
kClogging
before
clogging
in/hr
after
clogging
in/hr
k-Clean
1st
Backflush
in/hr
470.27
327.48
430.00
430.90
71.79
470.27
363.96
450.57
453.12
470.27
321.56
402.83
478.39
365.72
478.39
%
Removal
for
1st
Backflush
%
%
Removal
for
2nd
Backflush
%
k-Initial
before
clogging
in/hr
after
clogging
in/hr
72.42
470.27
297.53
429.36
429.48
76.31
76.39
81.47
83.86
470.27
361.04
434.27
435.45
67.04
68.12
432.32
54.65
74.48
470.27
327.48
433.75
448.33
74.42
84.63
444.45
448.33
69.88
73.32
478.39
325.41
436.11
438.37
72.36
73.84
299.13
420.58
456.42
67.75
87.74
478.39
349.57
450.57
452.83
78.40
80.16
478.39
358.26
416.93
424.67
48.84
55.28
478.39
342.34
416.33
441.19
54.38
72.65
476.65
286.29
441.73
441.86
81.65
81.73
476.65
360.41
452.83
459.18
79.51
84.97
476.65
300.13
464.96
469.05
93.38
95.69
476.65
297.22
446.66
437.70
83.28
78.29
476.65
318.43
444.04
452.41
79.39
84.68
476.65
334.47
468.74
469.97
94.44
95.30
487.62
327.63
457.14
480.15
80.95
95.33
487.62
372.81
472.74
476.02
87.04
89.90
487.62
317.66
464.51
460.65
86.40
84.13
487.62
300.19
460.95
458.74
85.77
84.59
487.62
296.24
475.55
481.27
93.69
96.68
487.62
305.15
469.51
471.81
90.07
91.34
67
kClean
1st
Backflush
in/hr
k-Clean
for
2nd
Backflush
in/hr
kClogging
REFERENCES
ASTM, (1997). Standard Practice for Sampling Aggregates. American Association State
Highway and Transportation Officials Standard. D75-03.
ASTM (1998). Standard Practice for Reducing Sample of Aggregates to Testing Size.
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