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OCR for page 69
CHAPTER 4
EXPERIMENTAL STUDIES
A number of experimental field and laboratory studies were necessary In order to
provide the data needed to develop the models used In the PAVDRN software. Permeability
measurements were obtained in the laboratory for open-graded laboratory and field asphalt
mixtures in order to obtain their coefficients of permeability. Mean texture depth
measurements were obtained for all of the pavement surfaces tested In the laboratory and field
using either the sand patch or a profiling method. Water film thickness measurements were
obtained In the laboratory with a color-~ndicat~ng gauge and a point gauge. The color
ndicat~g gauge was used exclusively In the field for water film thickness measurements.
The Indoor artificial rainfall simulator at Penn State was used in the laboratory to
determine ManIiing's n for porous asphalt surfaces and to extend the existing data on Portland
cement concrete surfaces to longer flow paths as required for PAVDRN.
In the field, full-scale skid testing measurements were needed to extend the
hydroplaning model to porous pavement surfaces and to verify the effect of Portland cement
concrete grooving on hydroplane g speed. These data were obtained by conducting filll-scale
skid test measurements on porous asphalt surfaces installed at the Penn State Pavement
Durability Research Facility. Full-scale skid testing was also performed on grooved PCC
surfaces at the Wallops Flight Facility. The fulI-scale held skid testing required measurements
69
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at different speeds on the surfaces flooded with water at different fihn thicknesses. The test
facilities, test melons, and test results are discussed In this chapter.
TEST FACILITIES
Indoor Artificial Rain Facility
The pavement test surfaces were formed ~ a rectangular channel that was 0.30 m wide
and 7~3 m long. The sides of the channel were formed by two BO-mm by 160-mm steel angles
that were mounted 0.30 m apart, as shown ~ figure 15. To complete the channel, the steel
angles and 20-mm thick sheets of plywood were bolted to the top flange of a 7.3-m wide flange
WI2x53 steed beam as shown In figure 16. A jacking system allowed the longitudinal slope of
the beam to be adjusted to provide a range of slopes. The porous asphalt concrete and Portland
cement concrete were placed In the channel, providing the test surfaces for measuring
Mami'ng's n.
Artificial rainfall was generated with a series of nozzles placed above the test surface,
as shown In figure 17. Extensive evaluations were performed previously to calibrate the
rainfall rate and to select appropriate nozzles, spray angles, nozzle distances from the channel
pressure settings, etc., to ensure that the rainfall rate was uniform over the entire surface (35).
Consequently, the procedures and testing equipment developed previously were used for this
study (299.
70
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Plywood
base
Porous
asphalt
mixture
Steel angle
to form sides
. Ad. ~
'~-~-~'J-~-~-~-~'~'~-~- - -~'~-~-~'~-~-~'~-~
ke%-%'~'~'~'~'~'%-~-~-~-~-~-~-~-~-~-%'%- - -~-~-~-~-~.
'~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~d
i..~.~-.~-.~.~-.~.~.~-.~.~-.~-.~-~-.~-.~.~.~-.~-.~-.~-.~-.~-.~-.~-~-~1
Steel"~" section
to support base
and mixture
a,
,
Note: Elevation of one end of steel
beam can be adjusted to change
longitudinal slope of drainage surface.
Figure 15. Cross-section of pavement used in laboratory rainfall simulator.
71
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Figure 16. Overall view of test channel used with laboratory rainfall simulator.
72
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~ : ~.,~ ~ ~ ~ ~ ~:::::: :~: ~i: :~: :::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::
::: ~:~::~: ~:~::~:::~:::::: If:
Figure 17. Laboratory rainfall simulator.
73
_ ~
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The channel was limited In length to 7.3 m. With this length and the maximum rainfall
rate, the largest Reynold's number that could be generated was approximately 130. However,
in this study it was necessary to measure WET values in flow regimes with Reynold's numbers
greater than 140. Since the maximum rainfall intensity was 75 mm/in, it was necessary to
effectively increase the drainage path lengths to achieve higher Reynold's numbers. This was
done by introducing a flow at the top of the channel so that the channel represented the last
7.3-m segment of a longer flow path. For example, to create a 14.6-m long flow path, the
flow that would be accumulated over the first 7.3-m segment was introduced at the top of the
channel, effectively making the channel act as the last 7.3-m segment of a 14.6-m long flow
path.
The flow introduced at the top of the channel was commensurate with the rainfalls rate
on the channel, adjusted for non-turbulent conditions in the first 0.5 m of flow. A small
adjustment in the introduced flow rate, as calculated on the basis of the rainfall rate, was
necessary because the turbulence caused by pelting raindrops impede flow. Approximately 0.5
m was required to develop fully turbulent flow, causing the actual flow to be greater than
under conditions where the flow on the entire 7.3-m channel length was filly turbulent. This
phenomenon has been observed by others when analyzing the short, sudden rise in flow at the
end of rainfall-runoff hydrographs (39J . The adjustment was determined experimentally by
measuring the flow at the end of the channel for different rainfall rates. The flow was
introduced at the top of the channel in a gentle spray applied directly onto the concrete surface
in the channel.
74
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For the porous asphalt mixtures, the How through the mixture had to be determined and
evaluated e A distribution box with a baffle was placed at the top of the channel to provide a
base flow through the porous mixes. The bottom of the channel was sealed to a depth of 12
mm below the top of the surface, effectively forming a dam to prevent ~awdown effects of the
flow through the porous asphalt. If the bottom was left completely open, the water surface
profile would draw down dramatically at the end of the channel, which would lessen the length
of the channel that could be used for experimentation. This arrangement is shown in figure 18.
Production and Placement of Porous Mixes
Three porous mixes were tested In ache laboratory under artificial rainfall. Each mixture
was designed to yield a different mean texture depth and air-void content. Attempts to place
hot-mixed asphalt in the channel were not successful, and instead, a slow-setting epoxy was
used as the binder for these mixtures by replacing the asphalt binder on a vol~netric basis.
The epoxy had a curing tune of six hours, which allowed for an adequate time to place and
compact the mixes.
A number of trial mixtures were prepared to obtain a range In air void content and
MTD. The composition of the resulting three porous asphalt mixtures placed in the laboratory
is shown in table 8, and the gradations are presented in figure 19. The mixes were prepared
from a blend of two coarse aggregates (PennnOT gradation IB and 2B, bow Innestone,
retained on No. 4 sieve) and washed glacial sand (siliceous, passing No. 4 sieve). Norrunal
75
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·1
a)
~ -
~ -
~ a)
~ ~ -
co o
-
an al
:=
ce
m cats
.E
~ 3
~ O
O ~
m .=
it'
\
\
\
ce
a, ma,,
oEl
!
I;
,1~
x
.
-
Q
In
In
3
2
o
ID 3
Cal o
(D-
In ~
~ o
CO °
Figure 18. Cross-section of flow for porous asphalt sections In laboratory
76
.
OCR for page 77
1
.~
i 1 1
; 1 i I ,
Ad ~
~ 2
X ~
an Y
CD CO
~_
oh an oh
: I l
: 1
1 ~
m
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ye
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Q
1 1
Con
~2
o
o
Q
J
1
1
_ Al ~
cry E
-
_ ~ ~
cad E
E
_ _
., E
1~''-~'- 1-..t
-ant.
~-.~ -A
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f 1 l l =~- 5=
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o o o o ~
CO C ~
o o o o
0 ~ ac
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0 0 0
~ u,
6u!ssed o/O
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%,
`. .
-\,
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Figure 19. Gradations of laboratory and field porous asphalt mr~tures.
77
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Table 8. Mixture designs for porous asphalt laboratory mixes.
Component Mixture
(% by total weight
of aggregate) A B C
2B Aggregate ~44
IB Aggregate 75 75 34
Washed Sand 19 19 20
Hydrated Lune 6 6 2
Epoxy (%wt. of 7 7 5.5
total mix)
maximum size is 9 and IS mm for IB and 2B aggregates, respectively. Hydrated lime was
added to thicken the epoxy and prevent drainage of the epoxy from the mixture. Mixture A
was designed using the guidelines and design process as outlined for open-graded friction
courses as published by NCHRP (109. This mixture was placed by hand, resulting in a very
high a~-void content, as illustrated In table 8.
Mixtures B and C were placed with a vibratory compactor; the gradations and
max~m~,rn aggregate size were selected to account for the increased compaction and to give a
range in air voids and MTD. The compactor, developed as part of this study, consisted of a
0.30-m square by 25.4-mm thick steel plate with an air vibrator mounted on top of the plate.
The vibrator is used commercially for applications such as vibrating granular materials from
storage bins. It is rated at 2,400 cycles per minute with 7.2 kN of applied force per cycle.
The entire assembly weighed 580 N. A photograph of the assembly is shown In figure 20.
78
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Figure 20. Photograph of vibratory compactor.
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texture depths of mixtures A, B. and C are visibly different and fall within the expected range
of ~ mm (0.04 in) to 3 mm (0. 12 Ins. Profile traces were used to calculate estimated texture
depth (ETD) according to ASTM E IS45-96, "Standard Method for Measuring Surface
Macrotexture Depth Using a Volumetric Technique," ISO standard as described In figure 29
(421. The results are presented in table ~ ~ . The sand patch measurements on the original
surface are suspect, especially for mixture A. The profile measurements were difficult to
obtain because the probe constantly stalled In the deep voids. Based on these facts, sand patch
measurements on replicates of the surface are the recommended technique for making texture
measurements even though it may not be convenient for field testing, particularly on highly
trafficked pavements. Texture measurements made at the Penn State Pavement Durability
Research Facility are found in table 12.
Table 11. Texture Kept measurements on laboratory porous asphalt sections.
Distance MTD Values (mm)
along
channel (m) Mix A Mix B MLX C
0.3 1.45 1.04 2.34
1.5 1.60 -- -
3.2 2.13 1.07 2.24
3.6 1.57 1.45 -
4.8 - 1.24 1.98
6.3
1.47 1.93
Average 1.70 1.24 2.13
Sand patch directly on 5.1 1.9 2.3
surface, Average (mm)
MTD estimated from
profile measurements 2.54 2.26 2.92
directly on surface (see
figure 29)
106
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Table 12. Sand patch data obtained at the Penn State Pavement Durability Research Facility.
_
Mean Texture Depth
Sand Patch Diameter (rnm)
(in)
MixStation ~2 3 4 Average
Station Section
Average Average
1105 149.2 136.5 139.7 139.7 1411.55
75 139.7 139.7 146.1 136.5 1401.60
45 146.1 146.1 146.1 139.7 1441.55
15 152.4 158.8 158.8 158.8 157 145 1.27 1.5
2 105 88.9 95.3 88.9 88.9 90 3.66
75 95.3 95.3 88.9 88.9 92 3.66
101.6 101.6 101.6 88.9 98 3.12
15 88.9 88.9 82.6 82.6 85 91 4.11
3 105 133.4 133.4 133.4 136.5 134 1.73
75 136.5 139.7 133.4 139.7 137 1.65
45 146.1 146.1 139.7 139.7 142 1.55
15 146.1 127.0 139.7 146.1 139 138 1.60 1.6
4 105 165.1 165.1 158.8 168.3 164 1.14
75 177.8 165.1 171.5 177.8 173 1.04
177.8 165.1 177.8 177.8 174 1.02
15 171.5 158.8 165
166 169 1.12 1.1
Full-Scale Skid Testing
Full-scale skid testing was done at the Penn State Pavement Durability Research
Facility and at the Wallops Flight Facility. The results of the testing performed at the Penn
107
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State Durability Research Facility are presented In figures 33 through 36 for the four test
sections. A great effort was required to obtain these results. The sections were dammed along
their side and flooded (one section at a tune) as described previously In this chapter. The skid
trailer was driven at different speeds down the track, and the tire, a bald ASTM E 524-88
("Standard Specification for Standard Smooth Tire for pavement Skid-Resistance Tests") tire,
was locked over the flooded middle portion of the section. Water film Sickness measurements
were taken with the color-~ndicat~ng gauge at intervals along the section immediately before
each test as described previously. This resulted in nearly 50 sets of skid resistance-water film
thickness data. In general, relatively uniform water film measurements were obtained, and
only a few of the data sets were discarded. Analog traces of wheel friction recorded by the
tester were examined for anomalous data. In order to obtain a zero thickness value of skid
resistance, the wheel of the trailer was locked on each section with no flooding but with a
damp surface. In general, replicate runs were made at each water film thickness and speed.
Although there is considerable variability In the data, several conclusions can be drawn
from the test results. For the water film thicknesses that were tested, the skid resistance values
were less than the "zero thickness" values. For each section, the skid resistance decreased as
the water film thickness Increased. However, the skid resistance typically reached a minimum
and then unexpectedly increased with increasing water film thickness. After some thought,
this was considered reasonable, explained by the "plough~ng" effect of the wave of water
pushed by the locked tire. Minim~:nn skid resistance values were in the range of four to ten
depending on the test section. Hydroplaning occurred on all of the test sections at 60 and 90
Inch when the water film thickness became high.
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1
· Flooded pavement, 30 km/in
· Flooded pavement, 60 km/in
· Flooded pavement, 90 km/in
0 Wet pavement, 60 km/in
1l 0 Flooded pavement, ribbed tire, 60 km/in I
60
50
Z~n
a)
Q
~ 30
~5
z
~ 20
In
40
10
l
3° km/in |
\ ~
~60 km/in
l
\ 4~
|9o km/in |
~ -
. .
i
0.0 5.0 1 0.0 1 5.0
Water film thickness, mm
Figure 33. Skid resistance measurements at ache Penn State Pavement Durability Research
Facility, mixture I.
109
OCR for page 110
Flooded pavement, 30 km/in
· Flooded pavement, 60 km/in
· Flooded pavement, 90 km/in
0 Wet pavement, 60 km/in
ll
l
50
45
40
z 35
; 30
Q
z
co 1 5
25
20
10
5
O
\ \/|30km/h| /
.~ ~ 1
~ \ / L
\ ~.
\ ~160 km/in| '~
. ~ ~
0.0
5.0 1 0.0 1 5.0 20.0
Water film thickness, mm
,~
Figure 34. Skid resistance measurements at the Penn State Pavement Durability Research
Facility, mixture 2.
~0
OCR for page 111
60
50
Z 40
Q
30
~ 20
cn
10
O
0.0
l
Flooded pavement, 30 km/in
· Floodecl pavement, 60 km/in
· Flooded pavement, 90 km/in
o Wet pavement, 60 km/in
\~ /
-
~ 130 km/in 1
60 km/in I
1
/ 90 km/in ~
-
-
. !
l
l
-
5.0 1 0.0 1 5.0 20.0
Water film thickness, mm
Figure 35. Skid resistance measurements at the Penn State Pavement Durability Research
Facility, mixture 3
.
OCR for page 112
· Flooded pavement, 30 km/in
· Flooded pavement, 60 km/in
· Flooded pavement, 90 km/in
o Wet pavement, 60 km/in
70
60
z 50
~n
-
~ 40 ~
Q
z 30
~5
._
C'' 20
10
o
/13° km/in| ~
~ my, ,/~k ~
~ :
! . . ~I 1
0-0
5.0 1 0.0 1 5.0 20.0
Water film thickness, mm
Figure 36. Skid resistance measurements at the Penn State Pavement Durability Research
Facility, mixture 4.
~2
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The results from the testing In the grooved and plain Portland cement concrete at the
Wallops Flight Facility are shown in table 13 arid figures 37 and 38. Quite surprisingly, the
skid resistance versus water film thickness relationship for the grooved versus the plain
Portland cement concrete surface was very similar when the mew texture depth is calculated
using the surface at the top of the grooves as the Datsun. Thus, although the grooves are a
definite aid ~ removing water from the pavement surface, they do little to relieve the water
film from beneath the tire. This effect is not apparent ~ the standard ASTM E 274 test as
illustrated In figures 34 and 35. ~ the opinion of the researchers, this is also the case with
porous asphalt surfaces. In other words, We main contribution offered by porous asphalt
pavement surfaces to the lowering of hydroplaning speed, even though it is a very significant
contribution, is ache Increase In the mean texture depth that these surfaces offer.
These findings do not agree with maIly practitioners who fee! that the grooving and
large texture ~ porous mixtures allows the water to drain from beneath the tire. Of course,
Me findings here are for the locked bald tire according to ASTM E 274, and the findings may
be different for more heavily loaded truck tires or grooved passenger tires.
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Table 13. Skid resistance test data obtained at the Wallops Flight Facility
.
Pavement Water Film (skp~ ehd) Skid Number Average
Brushed Concrete 12.5~' 60 14.8 14.8
12.5 75 9.6 9.6
12.5 90 6.1 6.1
12.5 82 7.1 7.1
12.5 100 4.6 4.6
Grooved Concrete 12.5 60 17.3 17.3
12.5 80 12.7 12.7
12.5
90
6.0
6.0
Brushed Concrete ASTM`2' 30 26.9
ASTM 30 31.5
ASTM 30 31.8 30.1
ASTM 60 18.6
ASTM 60 20.3
ASTM 60 24.2 23.2
ASTM 90 13.8
ASTM 90 15.3
ASTM 90 17.0 15.4
Grooved Concrete ASTM 30 30.9
ASTM 30 32.9 31.9
ASTM 60 22.4
ASTM 60 22.6
ASTM 60 46.2 30.4
ASTM 90 30.1 30.1
(')Flooded with water prior to testing.
(~'Water applied in front of tire in accordance with ASTM E 274.
~4
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II A S Tag Standennj T est L
B' Flood ed to ~ 2 m m
3 5
3 0
2 5
e
2 0
, 1 5
._
1 0
5
o
n
3 0 6 0 7 5
82 90 1 00
S peed k m/h
Ft ore 37. lest res ^ far pi ^ ccdlcretc sections at the TVillcqps Il1~1~ Facility
115
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MASTS Standard Test
~ Flooded to 12 mm
35
30
25
20
-
z
~ 15
._
oh
10
5
a
KEgg
1 ~
1 9
30
1
60
80
Speed, km/in
90
Figure 38. Test results for grooved concrete sections at the Wallops Flight Facility
116
.
Representative terms from entire chapter:
porous asphalt
