• I

EPARTMENTAL RESEARCH rt Number: 133-2

DEVELOPIElLl

OF A

TE XTURE PROFILE

TEXA HIG H WAY DEPA RTMENT

R

Development of a Texture

Profile Recorder

by

B. H. Ashkar

Research Report No. 133-2

for

A Pilot Study to Determine the Degree of Influence of Various Factors Pertaining to the Vehicle and the Pavement on Traffic

Accidents Under Wet Conditions

Research Project 1-8-69-133

Conducted by

Texas Highway Department Highway Design Division, Research Section

In Cooperation With The U. S. Department of Transportation

Federal Highway Administration Bureau of Public Roads

July~910

The OpIniOnS, findings, and conclusions expres sed in this pub­lication are those of the author and not necessarily those of the Bureau of Public Roads.

ACKNOWLEDGMENT

The research report herein was conducted under the super­vision of Mr. John F. Nixon. Engineer of Research. and the general supervision of Mr. Robert L. Lewis. Chief-Engineer of Highway De sign.

Mr. Paul R. Tutt, the study supervisor. contributed valuable guidance and suggestions throughout the development.

Acknowledgment is also given to Mr. Kenneth D. Hankins, Supervising Designing Research Engineer, who gave many hours of counsel and continuous advice.

iii

TABLE OF CONTENTS

Acknowledgments

List of Figures

Abstract.

Summary

Implementation Statement

Section I. State of the Art .

Section II. Description of Equipment

Section III. Calibration

Section IV. Procedure

Page

.iii

.. v

.. vi

. .vii

.viii

••••••••••••••• "1

.......... 2

. 5

........ 6

Section V. Analysis and Interpretation of Data .7

Section VI. Conclusions . . 13

References . 14

Appendix. . .. 15

iv

LIST OF FIGURES

Figure Page

1. Schem.atic Drawing of the Magnification Mechanism. ..... . . . . . . . . .3

2. Highly Sm.ooth Texture Profile . . . · . . . . . . .8

3. Sm.ooth Texture Profile . . .9 ..

4. Medium. Texture Profile . . . . . . . . · . . . 10

5. Rough Texture Profile . . . . . . . . . · . . . . . . . . 11

v

ABSTRACT

Development of a Texture Profile Recorder. Project Number HPR-1(9), 1-8-69-133 Investigator Research Agency

Sponsor

Date Started Status Estimated Completion:

Key Words

B. H. Ashkar Texas Highway Department

Texas Highway Department & Bureau of Public Roads December, 1969 September, 1968 Active August, 1971

Skid Resistance, Pavement Texture, Texture Measurement, Texture Measuring Equipment, Wet Pavement Accidents.

Surface texture is one of the main factors that influences the skid resistance and the possibility of hydrop1anning on wet weather pavements. Therefore, to determine a minimum level of ~kid resistance that should be possible on a particular pavement a knowledge of the drainage ability and characteristics of the roadway surface texture is required. Accordingly, a texture profile recorder which is a device designed to accurately record the local variations in surface geometry was fabricated.

vi

SUMMARY

It was concluded that a texture profile recorder for field

use should be a simple, portable, mechanical device that could be handled by one man and record enough length of the road to be representative of the area to be surveyed. The areas surveyed by the Texture Profile Recorder were the sites of the accidents occurring under wet conditions, a part of a research study aimed to investigate the elements involved in the roadway-tire-vehicle area with respect to wet weather accidents.

vii

IMPLEMENT ATION STATEMENT

The Texture Profile Recorder is a device which is portable

enough to be handled by one person and relatively inexpensive enough to be mas s produced. In the few tests performed with this instrument to date, the speed at which the pavement could be surveyed appeared to be reasonable. Therefore, it was decided that this machine be used in surveying wet weather accident sites as proposed in this project.

It should be remembered that future use of this instrument

will call for more improved data recording techniques. In order to achieve a more accurate and refined description of the pavement surface, an electronic sensor instead of the mechanical feeler can be used with the results stored on a magnetic tape.

viii

SECTION I

STATE OF THE ART:

Whlle the fine scale texture of the roadway surface is believed to be the predorninant factor deterrnining the skid resistance between tire and road at low speeds, the rate of decrease in coefficient is thought to partially depend on the larger scale texture. At speeds of the order of 80 rnph it would be help­ful to have sufficiently large and angular projections on the road surface (easily visible asperities) to deform the surface of the tire tread, even in the presence of a filrn of water on the surface as well as permitting the rapid disposal of the main bulk of water (Ref. 1, 2, 3, & 4). Thus, for a wet roadway to have a good skidding resistance at high speed, it is essential that the asperities on its surface be sufficiently large and angular or that the surface should have sufficient drainage character­istics. The extent to which either requirement is met in practice would require a knowledge of the texture of the road­way surface. Accordingly, a texture profile recorder similar to the instrument developed by the Southwest Research Institute at San Antonio was fabricated (Ref. 5).

1

SECTION II

DESCRIPTION OF EQUIPMENT:

The Texture Profile Recorder is an instrum.ent designed to scribe a m.agnified profile of the surface texture as a "feeler" is drawn across the surface. That is, the feeler is placed on the pavem.ent surface and as it is drawn over the surface texture irregularities, the vertical m.ovem.ent of the feeler is m.agnified through a linkage system.. The feeler-linkage system. is m.ounted on a platform. that m.oves horizontally on a base fram.e which is supported on the pavem.ent by four leveling screws. The m.ovem.ent of the feeler-linkage system. results in a duplicated, but m.agnified, texture profile which is scribed on a paper chart. Also, the' 'upward" vertical excursions are recorded on a counter of which the counter reading, at any tim.e, is the cum.ulative vertical peak heights of the surface texture through the length transversed by the feeler.

The Texture Profile Recorder consists of the following:

1. The m.ain m.etal base fram.e which is supported by 2-1/2" leveling screws. The rectangular base fram.e is 36" long and 12" wide and consists of a m.etal angle m.em.ber at each short end connected to each other by two steel shafts.

2. A stainless steel platform. which is m.ounted on the two shafts in such a way that it m.oves through a rack m.ach­anism. from. one end of the fram.e to the other. This m.ovem.ent is generated by a m.otor driven worm.-gear with an adjustable rotational speed.

3. Mounted on the platform. is the texture recording m.ech­anism. that consists of the following:

a. The pavem.ent feeler which is connected directly, through a linkage system., to a spring loaded stylus that rests on a pressure sensitive chart paper as shown in the schem.atic drawing in Fig. (1).

b. An accum.ulative digital counter which is linked to the spring loaded stylus arm. by a nylon cord and a one-

2

hm

01

it" h t

II

ht - Height of the asperity

hm- Magnified Height of the asperity is scri bed on the paper strip chart.

Magni fication Ratio:: hm =( X I ... Y I) (--Y-) h t X + Y YI

• ill • t

XI

~StYIUS

~inge

cg~ l(Hinge

-"'::s , \J~

l l l Feeler '" Y ;. x?I

e - Optimum Angle of the feeler.

I

FIGURE I Schematic Drawing of fhe Magnification Mechanism

. .

way clutch. A reading of 29 digits on this counter represents one inch of accumulative vertical movement of the feeler. This interpretation of the counter reading is explained in Sec. III. on Calibration .

4

SECTION III

CALIBRATION:

The Texture Profile Recorder has been calibrated for two different aspects of measurements as follows:

1) The local variations in the surface geometry are recorded on the paper strip chart with a magnification ratio. This

magnification ratio was found by using several different artifically textured wooden boards such that the heights of the asperties were known. The texture profile recorder was then tested on these calibration specimens and the resulting profile was measured and correlated with the known specimen heights. The magnification ratio was found to be 3.2. The two calibration specimens used are shown in Figures A- 1 and A- 2 of the Appendix. A sche­matic drawing of the magnification mechanism is shown in . (l).

It should also be remembered that the optimum angle and sharpness of the feeler shown in Fig. (1) have been selected by trial and error to allow the feeler to transverse the pavement surface without lodging and to produce an accurate reproduction of the surface texture profile at the same time.

2} After the optimum angle of the pavement feeler was determined by trial and error as mentioned previously, the digital counter was mounted such that when the feeler is moved one inch vertically the counter yields a «constant". The digital counter was calibrated using the various artifically textured wooden boards, as mentioned previously, such that the heights of the asperities were known. The Texture Profile Recorder was then tested on these calibration specimens and the resulting counter reading was divided by the known heights of the asperities to obtain the counter reading constant. This constant was found to be equal to 29. Note that the accumulative height of peaks is reported in inches.

5

. ..

SECTION IV

PROCEDURE:

In the field, the texture profile recorder is placed on the . pavement surface desired and the spring loaded stylus is released so that it encounters the paper strip chart. The counter is initialized to zero. The motor switch is turned on thus moving the feeler-stylus network horizontally along the frame thereby recording the magnified profile of the pavement surface texture. The counter reading is recorded (generally written on the chart paper) after which the paper strip chart is removed, identified, and returned to the office for inter­pretation.

It is noteworthy that the movement of the stylus-feeler network system is generated by a motor driven worm-gear with an adjustable speed. Accordingly, the minimum and maxi­mum periods of time required to make one texture measure­ment are 15 seconds and 45 seconds respectively. The change in speed at which the stylus assembly is driven has proven to have a very negligible effect on measurements taken. Also, the accuracy has proven to be very slightly affected by repeti­tive measurements.

The motor driving the stylus-feeler network is a small 1/8 HP, 12 volt, fan type motor that derives its power from the battary of the accompaning vehicle. In case of a motor mal­function, battery failure, or lack of an electrical power source, the texture profile recorder can be manually operated. The time required for a manual texture measurement is about 42 seconds.

6

SECTION V

ANALYSIS AND INTERPRETATION OF DATA:

The data obtained is processed in the following m.anner:

1. The counter reading is divided by the num.ber 29 to get the accum.u1ative height of the asperities in inches as explained previously.

2. Figures 2 through 5 illustrates the type of the chart paper

and actual grid size used. Also, shown are various texture profile traces with the peaks m.arked in sm.all circles. Arbitrarily, a peak has been defined as any m.agnified asperity with a m.inim.um. height of 1/16" and a m.axim.um. base length of 1/4" or any m.ultip1e set of these dim.insions.

3. The Average Peak Height is obtained by dividing the

accum.u1ative peak heights by the num.ber of peaks.

As an exam.p1e, the average peak height is obtained for each of the profiles in figures 2 through 5 as follows:

7

(X)

Scale:

Vertica I Horizontal

e .. 3.2". III or I"· 1'18 I"· I-

* Note: Counter Reading = 0 Number of Peaks= 0

..

* Above profile is a partial full size section taken from the original 21- 3" chart for which the number of peaks and the counter reading ore as shown above.

FIGURE 2. Highly Smooth Texture Profile

>D

Scale:

Vertical Horizontal

~

" 3.2" = I" or I" = 0/16 II' = I"

* Note: Counter Reading = 13 Number of Peaks = 25

..

* Above profile is a partial full size section token from the original 2'-3" chart from which the number of peaks and the counter ,reading are as s~own above.

FIGURE 3. Smoo t h Texture Profi Ie

(5

Scale: Vertical 3.2 11 = III or I" = 5/IS"

Horizontal III = I"

• .,.... 11 '

*' Note: Counter Reading = 30 Number of Peaks=40

-

* Above profile is a partial full size section taken from the original 2'-3" chart for which the number of peaks and the counter read ing are as shown above.

FIGURE 4. Medium Texture Profile

Scale: Vertica I 3.2" = III or III = 5/16

11

• II II Horlzonta I I = I

* Note: Counter Reading = 68 Number of Peoks = 59

..

* Above profile is a partial full size section taken from the original 2'-3" chart for which the number of peaks and the counter readi ng are as shown above.

FIGURE 5. Rough Texture Profile

C .R. Counter Reading A.P Accumulative Peak Heights (inches) N.P. NUrrlber of Peaks A.V.P.H.: Average Peak Height (inches)

C.R. A.P.H. N.P. A.V.P.H.

Figure *2 0 0.000 0 0,000

Figure 3 10 0.450 25 0.018

Figure 4 30 1.030 40 0.026

Figure 5 68 2.340 59 0.039

In the above table the Accurrlulative Peak Heights is obtained by dividing the Counter Reading by 29, while the Average Peak Height is the ratio of the Accurrlulative Peak Heights to the NUrrlber of Peaks.

*Note that the average peak height is set to zero when the nUrrlber of peaks is equal to zero. Matherrlatically, this is not correct because the average peak height is indeterrrlinate when the accurrlulative peak heights are divided by the nUrrlber •• zero" .

It should be remembered that the counter reading and the corresponding nUrrlber of peaks are rrleasured on a stretch of the pavement 2' - 3" in length.

It is felt that this rrlethod of analysis is a sirrlple rrleaSure used to assess the profile, which bears SOrrle relation to "texture depth" but also takes into account the nUrrlber of the asperities in the given length.

12

SECTION VI

CONCLUSl()NS:

To evaluate the effectiveness of surface texture as an important parameter ir..fluencing wet pavement skidding or hydroplanning, a textu!"e urofile recorder has been designed, developed, and fabricated,

The natun~ of this dev~'~e provides for two types cf measure­ments. Tbe <1igital me8.s'],:,ement is a rneasure of the coarseness of the roai\~, surface texture and hence of the texture depth which is a:lajor element that influences the rate of change of the wet coefficient of friction width speed. The other type of measurement is the graphical representation of the actual surface texture profile which can be used with proper engineering judgement to determine and compare the drainage abilities and characteristics of roadway surfaces.

13

L D. F. Moore

2. A. K. Stiffler

REFERENCES

- "Prediction of Skid-resistance Gradient and Drainage Characteristics for Pave­ments", HRB Report 1}.~, Washington, D.C., 1966

- "Relation Between We'!l' a.mi Physical

Properties of Roadstone.s' " HRB Special Report 101, Washington, D.C., 1969

3. Barbara E. Sabey - "Road Surface Texture and the C:hange in Skidding Resistance Width Speed", RRL Report No. 20, Ministry of Trans­port, Harmondsworth, England, 1966

4. S. A. H, z.nd J. W. Calloway

5. John M. Dale

- "Effect of Road SUlf?c~ Irr'zgularities and Structural Variability and Dynamic Pressures under Flexible Roads". RRL Report RL215, Ministry of Transport, Crowthrone, Berkshire, England, 1968

- "Development of Improved Pavement Marking Materials", NCHRP Report 45, Southwest Research Institute, San Antonio, Texas, 1967

14

• •

APPENDIX

15

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