Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
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
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
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
Figure 16. Overall view of test channel used with laboratory rainfall simulator. 72
~ : ~.,~ ~ ~ ~ ~ ~:::::: :~: ~i: :~: :::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::: ::: ~:~::~: ~:~::~:::~:::::: If: Figure 17. Laboratory rainfall simulator. 73 _ ~
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
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
·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 .
1 .~ i 1 1 ; 1 i I , Ad ~ ~ 2 X ~ an Y CD CO ~_ oh an oh : I l : 1 1 ~ m {D 0 ye Cal Q 1 1 Con ~2 o o Q J 1 1 _ Al ~ cry E - _ ~ ~ cad E E _ _ ., E 1~''-~'- 1-..t -ant. ~-.~ -A 1 1 1 1 1 - ~ 4g, f 1 l l =~- 5= 1 1 , .V ~ ~-~ ~. o o o o ~ CO C ~ o o o o 0 ~ ac ~ ~ E 0 0 0 ~ u, 6u!ssed o/O ~ E 1 _ _ _ %, `. . -\, ~: \\ (' \. \\~.t Figure 19. Gradations of laboratory and field porous asphalt mr~tures. 77
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
Figure 20. Photograph of vibratory compactor. 79
Cores were taken from the mixtures placed ~ the channel, and sections were removed for sand catch and profile testing. Air void content was determined for each mix ~ accordance with ASTM D 3203-88, "Standard Test Method for Percent Air Voids In Compacted Dense and Open Bituminous Paving Mixtures, and the results are shown In table 9 Table 9. Air voids In laboratory porous asphalt mixes. . Distance along Channel (m) Mix A Mix B % Air voids Mix C . 0.9 32.1 23.0 19.5 1.8 33.7 -- 20.2 2.7 34.6 23.7 23.4 3.7 29.0 22.9 19.8 4.6 32.5 22.9 21.8 5.5 32.5 22.5 20.0 6.4 7.3 Avg. 33.1 33.0 32.6 25.5 23.4 20.7 -20.8 Maxi Theoretical 2.460 2.467 2.504 Specific Gravity Outdoor Test Facilities Full-scale field skid testing was needed to verify the hydroplaning potential of the open graded asphalt concrete and grooved Portland cement concrete surfaces. Initially' this testing 80
was scheduled for the Wallops Flight Facility, and full-scale porous pavement sections were to be Installed at the facility. The facility offered a large flat area for testing at high speed with excellent support services. Unfortunately, after considerable planning, it became logistically impossible to place the test sections at the Wallops Flight Facility. After careful consideration of the alternatives, a decision was made to install four porous asphalt sections at the Penn State Pavement Durability Research Facility. However, it was decided that the research would continue to include the testing of the grooved and un-grooved PCC pavement at the Wallops Flight Facility, given that this pavement was in place, and no new construction would be required. Thus, the field skid testing was conducted on two PCC sections (broomed and broomed with groovings at the Wallops Flight Facility and on four open-graded asphalt concrete sections at the Penn State Pavement Durability Research Facility. Penn State Pavement Durability Research Facility The existing surface on which the mixes were to be placed required leveling because the surface was rutted and had a large cross-slope. The testing area was a tangent section at the facility with an average longitudinal slope of one percent, approximately 3.65 m (12 fit) wide and 200 m (600 It) long. First, the surface was milled to eliminate the existing cross- slope. Next, a typical PennDOT dense-graded ID-2 surface overlay (dense-graded with 9.0- mm top size) was placed over the test area to further eliminate any cross-slope and to provide a smooth testing area. The 2-m (6-ft) wide middle portion of the new overlay was then milled to a depth of approximately 40 mm (~.5 Ins. This, ~ essence, created a 2-m wide and 200-m long "bath tub" In which the porous asphalt mixes were placed, as presented In figure 21. 81
Width of flooded section Tubing fastened to / pavement surface \ / with silicone sealant V ~ /:...,-. Porous asphalt mixture / ~/~' / . IJi hi/ :, ~ // /,// '\\\ ~ Milled base '\,~ / /~ / vat ~ .. , . ~ , , ~ , ~ ~, i// ~ _ , ,,,~v-/ , // // / / / Dense-graded // / wearing course / / ~ Surface / seal New wearing course mix ` Vertical and horizontal surfaces sealed with asphalt cement to prevent leakage Figure 21. Schematic of test sections at the Penn State Pavement Durability Research Facility. 82
Four porous asphalt mixes were designed and placed at the test track facility with the cooperation of a local hot-mix contractor. A 40-m transition zone was established between each test section to allow the mixes to "run out" of the paver as the mix design was changed during the paving operation. This procedure was to ensure that the material In each test section was representative of the desired mix and not con~ninated with material from an adjacent section. Wooden 2-ft-by-4-ft boards were placed across the 4-m (12-ft) lane width at the end of each test section to separate the test section from the transition sections. These boards were later removed, leaving a small trench across the pavement at the ends of each test section. The gradation used for each of the mixtures is presented In table 10. The coarse aggregate was a local Innestone, and the sand was from a siliceous glacial river deposit (same material as used in the laboratory mixtures). The mixtures were designed to yield a range of air void contents and maximum aggregate size. Mixture ~ is based on the FHWA mixture design procedure for open-graded asphalt maces as outlined ~ NCHRP Synthesis 49 (109. Mixture 2 was based on a gradation reported by Hud~eston et al. (9J. Mixtures 3 and 4 were designed to represent Apical gradations as being performed by transportation agencies in France (161. Polyester fibers were added to each niLxture to men mire any tendency for drainage of the asphalt binder during construction. The binder content was selected in accordance with the standard design procedures detailed elsewhere, resulting In the binder contents shown In table 10 (101. 83
Table 10. Porous asphalt mix designs at the Penn State Pavement Durability Research Facility. Sieve Percent Passing Size Mix 1 Mix2 Mix3 Mix4 38 mm 100 100 100 100 25 mm 100 100 100 100 l9mm 100 99 100 100 13 mm 100 62 100 100 9mm 97 27 97 100 No. 4 28 6.9 29 76 No. 8 13 4.9 7.1 16 No.16 7.0 3.2 3.3 8.3 No. 30 4.4 2.2 2.4 5.5 No. 50 3.0 1.7 1.9 4.0 No. 100 2.0 1.4 1.5 2.9 No. 200 1.0 0.6 0.8 1.2 Asphalt Cement(%)l 6.5 5.0 6.0 6.5 Polyester Fibers(%)2 0.5 0.4 0.4 0.4 1Based on total weight of aggregate. 2Based on total weight of mixture. The objective of the testing at Penn State was to determine the effect of the water fihn thickness on the hydroplaning potential, which required that the test sections be flooded during the testing. Applying water in the conventional manger with the standard ASTM E 274-90 ("Standard Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire") skid Mailer would not give controlled or measurable water film thicknesses, and therefore, it was necessary to flood the test sections. Water was introduced ~ the trough formed by the four 84
wooden boards at the head of each section. The water was then allowed to flow over the entire length of the section, as depicted in figure 22. The depth of flow was controlled by adjusting the rate at which water was added to the trough. The longitudinal slope, approximately one percent, provided a reasonably uniform flow over the length of the section except at the beginning and end of the sections. The test sections were designed so that the flow of water through the pavement could be measured, thereby obtaining ~n-situ permeability measurements. This proved impractical because, in spite of being sealed with hot asphalt cement, leaks occurred in the depressed section. Water film thickness measurements were obtained just prior to each skid test using a color-indicat~ng gauge as described later In this chapter. Sand patch and profile measurements were also acquired for each section, and cores were obtained for laboratory permeability testing. The skid tester used for this project is a Penn State design, In which a s~ngle-whee! trailer is affixed to the rear of a modified heavy-duty pickup truck. The tester, commonly referred to as the s~gle-whee} skid tester, incorporates a s~x-force transducer into its design. This enables horizontal, vertical, and side force measurements. For this project, the s~ngle whee} skid tester was mounted in the center of the pickup to eliminate the effects of the truck tires on the waterfihn thickness. The testing was performed in accordance with ASTM Standard E 274. A photograph of the tester can be found in figure 23. 85
Figure 22. Introduction of water onto test section at the Penn State Pavement Durability Research Facility. 86
I: ::: :-:: ::: :: ::: :-:::: :::.::: :: :: :. ~ : : :-: .: ::: ::: ::::: I: ~ ::::: ::: :::: :: :::: :~: :: :: E .. .% A... ... ~ . ~ hi,. . ~ > . .. . . ·. ~,. ~. .. ~ .. ~ .~. ~ ..~ . Figure 23. Skid test in progress at the Penn State Pavement Durability Research Facility 87
Wallops Flight Facility The testing at the Wallops Flight Facility was performed in much the same mater as at Penn State. The sections were dammed, and water was flooded over the sections. Unforhmately, the water fiLn thickness was not as controlled as at the Penn State site, and only one water film thickness was reliably obtained. A photograph of a test in progress is shown in figure 24, and a photograph of the Portland cement concrete surface is presented in figure 25. MEASUREMENT TECHNIQUES Measurement of Water Film Thickness A point gauge was used to measure water film thicknesses in the laboratory (299. A point gauge is a pouted probe that is lowered from a stand until it comes in contact with the water surface. WET measurements were made at 0.3-m increments along the length of the channel. Three measurements, located at the m~-width and at the quarter-widths of the surface, were obtained at 0.3-m increments along the length of the surface. The three measurements were then averaged to obtain one WFT measurement for each 0.30-m increment along the length of the surface. 88
Figure 24. Test in progress at the Wallops Flight Facility. 89
Figure 25. Grooved concrete surface at the Wallops Flight Facility. 90
below the top of the asperities of the aggregates. The use of a flow date is discussed by Reed et al. (29) . To obtain the data, a 25-mm~iameter metal disc was first placed on the pavement surface at the measurement location, and a point gauge reading was obtained on the top of the disc. Next, a reading was taken on the surface of the flowing water, and the thickness of the washer plus one MID were subtracted from the point gauge reading on the water surface to obtain the flow depth. The method is illustrated In figure 26. In order to relate the hydroplaning speed to the water fihn thickness and to validate the water film thickness model, the water film thicknesses had to be measured In the field and In the laboratory during rainfall. The point gauge and other devices available for making these measurements were judged unacceptable for field use because the measurements are slow and tedious to perform and cannot be obtained during rainfall. Therefore, alternate procedures for measuring the WET were considered. The gauge that was ultimately adopted consists of a sheet-metal fixture bent In the form ot an Inverted "U." as shown In figure 27. The legs of the U are approximately SO mm high, and spaced 30 mm apart. The fixture is approximately 150 mm In length. To make a water film thickness measurement, the "legs" of the fixture were coated with a paint-like coating that changes color when wet. To obtain the data, the fixture was placed on the pavement with its "legs" immersed In the water fiIrn. The water film thickness is then determined as the dunension over which the coating changes color. 91
Note: Measured water film thickness is the difference betvveen point gauge reading on water film and washer plus thickness of washer as follows: WFT = (RPG' - RPGw3 + tw where WFT = Water film thickness RAG, = Reading of point gauge in contact with surface of water film RPGW = Reading of point gauge in contact with top of metal disk t`N = Thickness of metal disk Total flow depth, y RPG' Water film thickness, / WFT / ~ Ll t' ;; ~iVi . ~ MTD / ~ P_ , ~ ~ W Pavement surface ~ Figure 26. Measurement of water film thickness with point gauge on a porous asphalt surface In laboratory. 92
Sheet metal frame on pavement surface Top of water film Water-sensitive coating Height of coating with color change ~ Pavement surface 1. Figure 27. Schematic of the color-indicatillg water film thickness gauge. 93
The coating is initially yellow but turns bright red when it comes in contact with water. The device was calibrated in the laboratory by comparing water depths from the Portland cement concrete surface measured with the point gauge and the color-~ndicating gauge. The water film thickness measurements were obtained with a point gauge and with the color indicatiIlg gauge. The water film thickness values measured with the color-indicat~ng gauge were larger than the water fiDn thickness values measured with the point gauge because water "wicks" up the coating when the coating is wet. Sixty pairs of data points were obtained In the laboratory, with the color-~ndicating gauge and the pout gauge. A regression of the data points resulted In the relationship: KK = 0.907 WFT + 3.~1 where KK WET Color-indicating gauge reading (mm) Actual water fiDn thickness value (mm) (26) with a correlation coefficient (R2) of O.85. This relationship is displayed graphically ~ figure 28. The color-~ndicat~g gauge was used for all of the held testing conducted at the Penn State Pavement Durability Research Facility and the Wallops Flight Facility. 94
10 ~9 ·3 J _ In ~ ~ _ ~ a) E ~ V CD by oh _ 5 a) ~ ._ Y ~ e_ _ ._ O o ._ Cal a) - 8 7 t! 4 Cow o 3 2 1 o lo -A °~a9' KK= O.907W~+3.81 R2 = O.851 . . . . . . . . . . . . . . . . . j , _ o 1 2 3 4 5 Water film thickness, WET, measured with point gauge, mm Figure 28. Correlation of water film thickness measurements obtained with the color indicating gauge and point gauge. 95
Measurement of Surface Tenure A portable texture measuring device was used to perform surface texture profiles for the laboratory and field mixtures. The device produces an analog profile of the surface that can be digitized and analyzed statistically. The device is essentially a probe that is moved along Me surface of the pavement and is described in detail elsewhere (409. A motor and appropriate electronic circuitry cause the probe to follow the pavement surface as the probe is moved horizontally over the surface. The procedure for using this device has been standardized by ASTM as Designation E IS45-96, "Standard Practice for Calculating Mean Profile Depth." The procedure requires that two profile segments, each 100 mm in length, be obtained. The mean profile depth (MPD) for each of these profile segments is calculated by regressing the profile depth versus profile length, and the two values are then averaged to obtain the mean profile depth. The process is s~nnTnar~zed In figure 29. The mean profile depth can be used to estimate the mean texture depth (ETD in equation 27) by a linear transformation (411: MTD=ETD = 0.2 ~ 0.8 MPD (mm) The mean texture depth is by definition obtained with the "sand patch" (volumetric) (27) method, ASTM E 965, "Standard Method for Measuring Surface Macrotexture Depth Using a Volumetric Technique." The ETD is an estimate of the MTD. Mean texture depths were also obtained from sand patch testing (ASTM E 965) performed in the field and on laboratory 96
I . Profile measurements Calibrate the measuring system (when appropriate) and measure the profile of the surface. 2. Handling of invalid readings Readings of this profile that are invalid (drop-outs) shall be eliminated or corrected. 3. High-pass fiZtenug Unless slope suppression according to point 6 in the following is used, high-pass filtering should be performed. It consists of removing spatial frequency component that are below the specified passband. 4. Low-pass fiItenng Remove frequency components that are above the specified passband. This can be accomplished either by analog filtering or averaging of adjacent samples, or automatically met through the performance of the sensor. 5. Baseline limiting Pick out a part of the profile that has a satisfactory baseline. 6. Slope suppression The slope will be suppressed by the calculation of the regression Ime and subsequent subtraction of this line. An alternative is to apply appropriate high-pass filtering (see point 3 above). 7. Peal determination The peak value of the profile over the baseline length is detected. 8. MPD determination The mean profile depth (MPD) is calculated as the peak according to pout 7 above minus the profile average, which will be O according to points 3 or 6 above. 9. ETD calculation The MPD value is transformed to an estimated texture depth (ETD) by applying a transformation equation, ETD = 0.2 ~ 0.8 MPD. 10. Averaging of MPD and ETD values Individual values measured on a site or a number of laboratory samples are averaged. This includes the calculation of the standard deviation. Figure 29. Steps in determining texture depth using the profiling method (42~. 97
samples. In this test, a known weight of glass beads is placed on the pavement surface and spread by hand with a rigid scrapper until the surface voids are filled. The area of the resulting "patch" of glass beads is related to the MID. Measurement of Mann~ng's n Mann~ng's n for the Portland cement concrete surface and for the porous asphalt surface was calculated by measuring the water film thickness on these surfaces with varying rainfall rates arid surface slope. The theoretical base for the calculation is given in Appendix C, and the results of the calculations are presented in Chapter 3. Measurement of Permeability The static coefficient of permeability of porous asphalt concrete mixtures is a necessary input for the PAVDRN model. The customary procedure for measuring the In situ permeability of porous asphalt mixtures is to use an outflow meter. The outflow meter does not give permeability values ~ ndamental units, but instead provides an empirical measurement of the permeabili~ n terms of the quantity of flow per unit of time. Because the flow is unconfined In a radial direction, it is not possible to calculate a coefficient of permeability from the coIlventional outflow meter. Further, for OGAC, the flow is partially in the macrotexture and partially within the mix. Therefore, a direct measurement of static permeability was used for the mixtures that were tested as part of this project. Because of the 98
large coefficient of permeability of porous asphalt mixtures, a standard falling head parameter cannot be used. In order to obtain a measurable flow, a large quantity of water would be required and the rate of flow would be excessive, certainly in the turbulent region. Consequently, a drainage lag permeameter, originally described by Barker et al. (43) was used for the permeability measurements. The device, as shown in figure 30, consists of a tank, a sample container that confines the flow to the vertical direction, and a quick-release valve. Full thickness samples from the artificial rain facility were cut into squares approximately 80 mm by 80 mm in length and width, and sheet metal was expoxied to the sides of the samples to constrain the flow in the vertical direction. The permeability of the samples was measured in both the vertical and horizontal direction by testing samples oriented in both directions. Separate samples were used for each direction. This procedure ensured that there was no leakage around the periphery of the samples and that the flow occurred In the vertical direction. S~x-~nch cores were obtained from the test track facility, and the lower layer of hot mix was trimmed from the cores, yielding a section that consisted of only the permeable asphalt mixture. These cores were sealed around their circumference to confine the How to the vertical direction. The cores were then inserted ~ a 6-'n diameter sheet metal tube and sealed around their circumference using silicone sealant, ~ the same manner as the rectangular samples from the indoor rain facility. Once the cores were tested in the vertical direction, they were removed from the container, and rectangular-shaped sections for testing in the horizontal direction were sawn from the cores. These cores were tested in the same manner as the 99
Water height after time t' 1 - - Initial water height 7 r ~l / / \ . Sample Container Test specimen Perforated shelf \ - 1 1 Figure 30. Schematic of drainage lag permemneter. 100 Tank Quick opening valve 7
rectangular-shaped cores from the artificial rain facility. This procedure provided a vertical and horizontal coefficient of permeability for the field cores. All of the samples were vacuum-saturated prior to testing. The samples were immersed In a flooded transfer vessel to a level above the sheet metal containers and placed In a vacuum chamber. A vacuum was applied to die samples until they ceased bubbling, using techniques similar to those used In measuring the maximum specific gravity for asphalt concrete, as specified ASTM D 3203-94, "Standard Test Method for Percent Air Voids In Compacted Dense and Open Bituminous Paving Mixtures." Once the samples were saturated, they were placed In the tank, the quick opening value was opened, and the water draining from the tank was collected in a container during the tune interval when the water level in the tank Intersected successive points on the hook gauge. This provided sufficient data to calculate the coefficient of permeability in accordance with the equation reported by Barker (43), where: 276 ad h, k= A log h Q = k A h d 101 (28) (29)
where Q = Rate of flow (ft3/s) (1 ft3/s-0.028 m3/s) k = Coefficient of permeability A = Gross area of sample perpendicular to direction of flow (ft2) (1 ft2 = 0.093 m2) h = highs, Head loss at distance d In sample In direction of flow (ft) (1 It = 0.305m) Three tests were performed on each core In both the vertical and horizontal flow direction. Measured permeability values for the porous asphalt mixes from the field (mixtures 1 through 4) and the laboratory mixtures (mixtures A through C) ranged from 20 to 40 mm/s. Given the narrow range of ache measured values and the likelihood of reduced permeability resulting from plugging due to road detritus, the use of the drainage lag parameter is not recommended for routme testing or as a design procedure. TEST RESULTS Flow on Porous Asphall Sections In order to determine the surface flow rate for the porous mixtures it was necessary to determine the flow rate through each mixture that would saturate the mixture to a height of one MTD. This "base flow" was subtracted from the total flow to yield only the surface flow as illustrated ~ figure 3 I. In order to determine the base flow, a plot of flow depth versus total flow was constructed as shown in figure 32. These plots were prepared for each surface and 102
Rainfall Intensity, I . I l l' ~I I I 1. 1 1 1 1 _ Total AL surface _ . tl~lVV~-$ flow A A MA ]~--' ~ Base flow J" J Figure 3 1. Definition of base and surface flow in porous asphalt sections. 103
160 140 120 100 80 3 LL 60 40 20 o : _ 1 1 R2 = 0,99 F , . 1 . 0 5 10 15 20 25 Distance along channel, m Figure 32. Plot of total flow versus flow path to determine flow depth. 104
for each rainfall rate and slope. The base flow rate depended on the mix, the slope of the channel, and the rainfall intensity and varied from 1.5 ml/s for mixture B (25 mTn/h) to 53 ml/s for mixture A (75 mm/h). The use of the water film thickness values corrected for the base flow is discussed in detail in Appendix C where the development surface-specific equations for Manning's n is presented. Texture Measurements Texture measurements were made on the surfaces tested In the laboratory and field. The conventional sand patch technique causes problems with highly open mixtures because the glass beads flow into the internal voids in the mixture, giving a false value of texture depth. To overcome this problem, texture measurements were made on the laboratory porous mixtures using the conventional sand patch procedure on a cast of the surface. Texture depths were also estimated from profile measurements made on the original surfaces as presented in table 1 1. The casts, or replicates, were made by first placing silicone rubber on the original surface over an area of approximately 0.30 m by 0.30 m (1 ft by 1 ft). A plate was placed over the silicone rubber in order to force the rubber into the surface texture. Once the silicone had cured, it was removed and placed into a second form. A polyester casting resin was then poured over the surface of the silicone rubber and, on curing, separated from the rubber. The casting resin gave a positive replicate of the original surface. Casts were obtained from each porous asphalt surface, spaced at equal intervals down the length of the test surface that was 7.3 m long by 0.3 m wide, and sand patch measurements were made on the casts. The results of this procedure are shown In table ~ ~ . The mean 105
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
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
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. 108
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
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
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 .
· 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
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. ~3
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
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
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 .
