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SURFACE IRREGULARITIES, UNEVENNESS AND SURFACE DEFLECTION OF ROAD / HIGHWAY


CONTENT
                        
 SL No
             TYPE OF INSPECTION
 
 
     1DETERMINATION OF SURFACE IRREGULARITIES (UNEVENNESS) 
     2DETERMINATION OF SURFACE UNEVENNESS BY AUTOMATIC ROADUNEVENNESS RECORDER / ROUGHOMETER. 
     3DETERMINATION OF STRENGTHENING OF FLEXIBLE ROAD PAVEMENT USING BENKELMAN BEAM DEFLECTION TECHNIQUE. 
     4 CGRA       – DEFLECTION TEST PROCEDURE. 
     5 WASHO     – DEFLECTION TEST PROCEDURE. 



1. DETERMINATION OF SURFACE IRREGULARITIES (UNEVENNESS)
(IRC: SP: 11)

Object:

 Checking the surface unevenness using a straight edge and wedge.

 Apparatus:

 

1) 3-metre straight edge may be made of steel or seasoned hard wood with the

dimensions of 75mm wide and 125mm deep.

 

2) Graduated wedge – with a least count of at least 3mm.

 

Procedure:

 

For recording undulations in the longitudinal profile the straight edge is to be placed longitudinally parallel to the center line of the road measurements along two parallel lines may normally the sufficient for a single lane pavement and along three lines for the two lane pavement one additional line may be covered for each additional lane.

 

The straight edge has limitations for as regards the measurement of undulations at vertical curves. Additional templates may be made for this purpose especially if the curves are sharp.

 

The straight edge may be placed at the starting point, wedge inserted between it and the test surface where the gap is maximum and reading taken. The edge may then be slide by about ½ length i.e. 1.5m, and the wedge reading repeated. This process should be continued. The straight edge need not always be moved forward but may be moved backward and forward to record the maximum undulation existing at a location.

 

Locations with undulations in excess of the specified magnitude should be marked on the surface.

 

2. DETERMINATION OF SURFACE UNEVENNESS BY AUTOMATIC ROAD

UNEVENNESS RECORDER / ROUGHOMETER.

 

Automatic road unevenness recorder also known as BUMP INTEGRATOR or ROUGHOMETER gives speedily a quantitative integrated evaluation of surface irregularities on an electromagnetic counter. It comprises of a trailer of single wheel with a pneumatic tyre mounted on a chassis over which on integrating device is fitted.

The machine has a panel board fitted with two sets of electromagnetic counters for counting the uneven index value. The operating speed of the machine is 30 +/- ½ km/hr. A vehicle, usually a jeep, towed the machine and tyre pressure is 2.1 kg/cm3 (30 PSI).

 

Operation of the Bump integrator:

 

The Bump integrator should towed by a vehicle at a speed of 30 km/hr. A jeep with a canvass body is eminently suitable because it affords opportunities for the driver to keep a watch on the unit. The jeep should have sound suspension and damping system.  The tyres pressure in the rear wheels of the jeep should be same.

 

Before carrying out tests it is advisable to check the following items:

 

1) Integrator-cord terminals are in parked position i.e. the cord joining the integrator unit to the wheel axle should be fathered to the chassis in a non-operational position.

 

2) Tyre pressure is 2.1 kg/cm3 (30 PSI).

3) Level of fluid in dashpots is up to top mark of the dipstick (check and maintain level to 3 cms below the cylinder cap).

 

4) The electromagnetic counters are working in this is simply checked by running a short distance in operational mode.

 

5) All the bolts of the towing hitch and the bracket fixed with the towing vehicle are tight.

 

6) The caster-wheel adjustable jacks are removed from the Bump integrator.

 

7) Tyre pressure gauge and foot pump kept in towing vehicle.

 

8) The control-knob (if provided) of the wheel-revolution contact is screwed down to disengage the counter (this knob is located on top of the small box positioned where the wheel is bolted to the axle).

 

9) Spare like integrator cords, tool-kit, dipstick and fluid for the dashpots kept in the towing vehicle.

 

10) The relevant result-recording forms are also kept with the counter-board.

 

Comprehensive notes should be taken giving particulars of test length, width of road, type of surfacing and any other relevant details needed.

 

The machines driver over the test section of road at a speed of 30 +/- ½ kmph. Normallyin the nearside wheel track of the nearside lane (position of machine on road is dependent on purpose of measurements).

 

At the beginning of the section, the observer having set the counters to zero operates the on/off switch to on-position on the instrument board.

 

At the end of the section the observer changes over to the second set of counters, which are then set to zeros thus allowing continuous measurements. The readings of the integrating and wheel-revolution counters are entered. The test section should not be less than 500m. Bump integrator is run on two parallel lines (on each wheel track) on single lane carriageway and on three parallel lines (one on each wheel track and the third on the central line) for double lane carriageway.

 

Processing of results obtained with Bump integrator:

The results obtained with Bump integrator are the following for a test section over which it has been run to evaluate its riding quality:

 

i) Integrator value of irregularities in inches (BI counter reading).

 

ii) The number of wheel revolutions (wheel revolution counter).

 

Each set of readings (BI reading and corresponding number of wheel-revolutions) are required to be converted to the unevenness index value (UI value) in terms of cms/km.

 

The unevenness index value for the test section is arrived at by taking mean of UI values corresponding to the three sets of readings.

 

The unevenness index value is calculated by dividing the BI counter values (in cms.) by the distance traveled in kms.

    Integrator counter value (cms)

= ------------------------------------------

Distance traveled (km)

3.DETERMINATION OF STRENGTHENING OF FLEXIBLE ROAD

PAVEMENTS USING BENKELMAN BEAM DEFLECTION TECHNIQUE.

 

INTRODUCTION:

 

Ever since Benkelman devised the simple deflection beam for measurement of pavement surface deflection on WASHO test road in 1953, its use has become quite popular for evaluation of strengthening requirements of flexible pavements quickly and conveniently, Benkelman beam has been in use in India for more then a decade by different organizations. Since no uniform procedure was available for the design of flexible overlays by using the deflection technique, the specifications and standards committee of the IRC flat the necessity for preparing tentative guidelines.

 

Basic principles of the deflection method:

 

The deflection method is based on the concept that pavement sections, which have been conditioned by traffic, deform elastically under a load. The deformation of elastic deflection under given load depends upon the sub grade soil type and its conditions of moisture and compaction, thickness and quality of the pavement courses, drainage condition, pavement surface temperature etc. Extensive studies in other countries have shown that performance and life of flexible pavements are closely related to the pavement elastic deflection caused by the passage of wheel loads.

 

The Benkelman Beam measures pavement deflection under a wheel load. It consists of a slender beam 3.66m long pivoted at a distance of 2.44m, from the tip by suitable placing the probe between the dual wheels of a loaded truck; it is possible to measure the rebound and residual deflections of the pavement structure. While the rebound deflection is the one related to pavement performance, the residual deflection may be due to nonrecoverable deflection of the pavement or because of the influence of the deflection bowl on the front legs of the beam.

 

Procedure for deflection survey:

 

The deflection survey essentially consists of two sets of operation, namely,

 

i) conditions survey for collecting basic information about the road structure and based

on this demarcation of the road into sections of more or less equal performance, and

 

ii) actual deflection measurements.

 

i) Pavement condition survey: This phase of operation, which should precede the actual deflection measurements, consists primarily of visual observations supplemented by simple measurements for rut depth using a 3-meter straight edge. Based on these, the road should be classified into sections of equal performance with the criteria given as under.

 

 

 Classification

 Pavement conditions

 Good

 No cracking, rutting less than 10mm.

 Fair

 No cracking or cracking confined to single crack in wheel track with rutting between 10mm and 20mm.

 Poor

 Extensive cracking and/or rutting greater than 20mm.

 

 

 

As it is inexpedient to modify the overlay design at frequent intervals, it will be preferable if length of each section is kept minimum of 50m.

 

During condition survey, information should also be collected about drainage characteristics, topography, climatic condition and other relevant features. Test pits should be dug approximated every 250 –500m depending on the uniformity in performance or pavement structure to determine the thickness and composition of the pavement layers as also the sub grade soil characteristics. Where it is intended to compare the results with the CBR design method. CBR of the sub grade soil should also be determined on the lines recommended in IRC:37-1970.

 

The data collected at the condition survey should be recorded a proforma for which is suggested in below.

 

Format for the collection of field information during test.

Pavement condition survey.

 

 

@ This test may be conducted where it is desired to compare the overlay design with that given by CBR method. The test condition of the CBR specimens should be as recommended in IRC: 37.

 

ii) Deflection measurements:

 

In each road section of uniform performance minimum ten equidistant points should be marked in each lane of traffic for making the deflection observations in the outer wheel path. The interval between the points might vary from as low as 50m depending on the length of section under investigation. On roads with more than one lane, the points marked on adjacent lanes should be staggered. In the transverse direction, the measurement points should be 60cm from the pavement edge where the lane width is less than 3.5 and 90cm for wider lane width.

 

For measuring the pavement deflection, several procedures are available and fall under two main categories, (i) testing under static load, and (ii) testing under creep speed. For the purpose of these guidelines, either the CGRA procedure, which is based on testing under static load, or the WASHO procedure, based on creep load test may be made. In both cease methods; a standard truck having the rear axle equipped with dual tyres inflated to a pressure of 5.60 kg/cm2 and transmitting a load of 8170 kg is used for loading the pavement. During actual tests, the load and tyre pressure are maintained within a tolerance of +/- 1% and +/- 5% respectively.

 

The pavement temperature also influences deflections measured by Benkelman Beam.

 

For design purpose therefore, all deflection values should be related to a single common temperature, which is recommended to be 350C. Measurement made when the pavement temperature is other than 350C should be corrected in accordance with the procedure.

 

Pavement deflections are also affected by the seasonal variation in climate. For the purpose of these guidelines, it is intended that the pavement deflections should pertain to the period when the sub grade is at its weakest condition. In India, this period occurs  during the monsoon season and immediately thereafter. It is desirable to conduct the deflection measurements during such periods, but where this is not feasible a correction factor may be applied to the deflection value.

 

The deflection measurements and other information collected during the deflection survey should be recorded; for this a proforma is given below. This table also has columns for working out overlay thickness.

 

Format for recording and analysis of pavement deflection data.

 

 

Correction for temperature variations:

 

The stiffness of bituminous layers charges with temperature of the binder and consequently the surface deflections of a given pavement will vary depending on the temperature of the constituent bituminous layers. For purpose of design, therefore it is necessary that the measured deflection be corrected to a common standard temperature of 350C. Correction for temperature is not applicable in case of roads with thin bituminous surfacing (such as premix carpet or surface dressing over a non-bituminous base) since these are usually unaffected by changes in temperature. But temperature correction will be required for pavement having a substantial thickness of bituminous construction (i.e. minimum 40mm). Correction need not however be applied in the later case if road is subject to severe cracking or bituminous layer is substantially stripped.

 

The deflection pavement temperature relationship is linear above temperature of 300C. For convenience in the application of temperature correction it is recommended that deflection measurements should be taken when pavement temperature is greater than 300C.

 

Correction for temperature variation on deflection values for pavement temperature greater than 300C should be 0.0065mm for each degree centigrade change from standard temperature of 350C. The correction will be positive for pavement temperature lower than 350C and vice versa. For example if deflection is measured at 370C. The correction factor will be (2X0.0065 = 0.013mm). The correction should be subtracted from measured deflection to obtain corrected value corresponding to standard pavement temperature of 350C. The deflection measurement should take when pavement temperature is uniform and near about 350C, so measurements should be made during morning or evening hours.

 

In colder areas, the areas of altitude greater than 1000m where the average day temperature is less than 200C for more than 4 months, it is recommended that deflection measurements be made when ambient temperature is above 200C and no correction for temperature is required.

 

In cases, where temperature correction is required, pavement temperature should be measured, during deflection survey. The measurement should be made at depth of 40mm using short stem mercury thermometer. A hole of about 10mm dia and 40mm deep should be dug in pavement and filled with enough glycerol.

 

Correction for seasonal variation of deflection:

 

Since the pavement deflection is dependent upon change in the climatic season of the year, it is always desirable to take deflection reading during the season when the pavement is in its weakest condition, because permanent deformation and consequent pavement distress occurs in this period of the year. Since in India this period occurred during and after monsoon, deflection measurements should confined to this period only as far as possible. When deflections are measured during summer they require a correction factor (which is defined as the maximum deflection during or immediately after monsoon to that of minimum deflection in summer). This correction and other

factors due to large variations in subgrade soil composition and climatic conditions prevailing in India, it is difficult to give single value to this factor. It is, however, recommended that for dry clayey subgrade soils it may be taken as 2 where as for sandy subgrade it may be taken as 1.2 to 1.3.

 

Analysis of deflection data:

 

Deflection values as corrected above should be plotted against the chainages. For each section mean and standard deviation should be obtained using the following formula.

 

Sum of X

Mean deflection x = ------------

      n

 

Where            X = Individual deflections.

n = Number of measurements.

x = Mean deflection.

 

Sum of (X – x2)

Standard deviation (SD) = -----------------

   (n – 1)

 

Where            SD = Standard deviation

X = Individual deflection

n = Number of measurements

x = Mean deflection

 

Characteristic deflection = (x + SD)

 

Allowable deflection:

 

Based on the limited experience available in this country following tentative limits are set for allowable deflection corresponding to two methods of deflection measurements.

 

                             

 

*Allowable deflections where the deflections are

 measured by :

 Traffic Intensity

 CGRA method

 WASHO method

 

 250 - 450 commercial vehicles/day

     1.50 mm

     1.40 mm

 

 450 - 1500 commercial vehicles/day

     1.25 mm

     1.10 mm

 

 1500 - 4500 commercial vehicles/day

     1.00 mm

      0.80mm

 

 

 

Determination of overlay thickness:

 

Having obtained the characteristic deflection and permissible deflection for a given section, following relationship may be used for obtaining overlay thickness:

 

h = R log10 (x/y)

 

Where,           h = thickness of granular overlay in mm

x = characteristic deflection

y = allowable deflection

R = constant which may be taken as 550.

 

Evaluation of overlay thickness by CGRA curves:

The overlays to be provided can be assessed with the help of CGRA curves. The deflection having known and the traffic intensity known the overlay can be directly read on Y-axis. The various curves give the allowable deflection in the particular type of load for particular traffic intensity. The X-axis shows or read the observed deflection. The Yaxis reading will get us the thickness of overlays to be laid on the existing crust to meet the traffic demand.

 

CGRA – DEFLECTION TEST PROCEDURE.

 

Object:

This method of test covers a procedure for the determination of the static rebound deflection of pavement under a standardized axle load, tyre size, tyre spacing and tyre pressure.

 

Equipment:

 

The equipment shall include:-

1) Benkelman Beam:

 

a) Length of probe arm from pivot to probe point. - 244 cm.

 

b) Length of measurement arm from pivot to dial. - 122 cm.

 

c) Distance from pivot to front legs. - 25 cm.

 

d) Distance from pivot to rear legs. - 166 cm.

 

e) Lateral spacing of front support legs. - 33 cm.

 

2) A 5-ton truck is recommended as the reaction. The vehicle shall have an 8170 kg. rear axle load equally distributed in two wheels, equipped with dual tyres. The tyres shall be 10.00X20-12 ply inflated to a pressure of 5.60 kg/cm2. The use of tyres with tubes and rib treads is recommended.

 

3) Tyre pressure measuring gauge.

 

4) Thermometer (0 – 1000C) with 10 division.

 

5) A mandrel for making 4.5 cm deep hole in the pavement for temperature measurement. The diameter of the hole at the surface shall be (1.25 cm) and at bottom 1 cm.

 

Procedure:

 

1) The point on the pavement to be tested is selected and marked. For highways, the points are located 60 cm from the pavement edge if the lane width is less than 350 cm, 90 cm from pavement edge if lane width is 350 cm or more.

 

2) The dual wheels of the truck are centered above the selected point.

 

3) The probe of the Benkelman Beam is inserted between the duals and placed on the selected point.

 

4) The locking pin is removed from the beam and the legs are adjusted so that the plunger of the beam is in contact with the stem of the dial gauge. The beam pivot arms are checked for free movement.

 

5) The dial gauge is at approximately 1 cm. The initial reading is recorded when the rate of deformation of the pavement is equal or less than 0.025 mm per minute.

 

6) The truck is slowly driven a distance of 270 cm and stopped.

 

7) An intermediate reading is recorded when the rate of recovery of the pavement is equal to or less than 0.025 mm per minute.

 

8) The truck is driven forward a further 9 m.

 

9) The final reading is recorded when the rate of recovery of pavement is equal to or less than 0.025 mm per minute.

 

10) Pavement temperature is recorded at least once every hour, inserting thermometer in the standard hole and filling up the whole with water. At the same time air temperature isrecorded.

 

11) The tyre pressure is checked at two to three hour intervals during the day and adjusted to the standard, if necessary.

 

Calculations:

 

1) Subtract the final dial reading from the initial dial reading. Subtract the intermediate dial reading from the initial dial reading.

 

2) If the differential readings obtained compare within 0.025 mm, the actual deflection is twice the final differential reading.

 

3) If the differential readings obtained do not compare to 0.025 mm, twice the final differential dial reading represents the apparent pavement deflection.

 

4) Apparent deflections are corrected by means of the following formula:-

 

XT = XA + 2.91 Y

 

Where            XT = True pavement deflection.

XA = Apparent pavement deflection.

Y = Vertical movement of the front legs i.e., twice the difference

       between the final and intermediate dial readings.

 

WASHO – DEFLECTION TEST PROCEDURE.

 

Equipment:

 

1) Benkelman Beam

 

2) 5-tonne truck with 8170 kg. rear axle load equally distributed on the two wheels. The 10X20-12 ply tyres should be inflated to 5.60-kg/cm2 pressure.

 

3) Tyre pressure measuring gauge & other equipments are given in CGRA test procedure.

 

Procedure:

 

1) The point on the pavement where deflection measurement is desired will be selected

and marked.

 

2) Move the truck so that its rear wheel is about 1.2m behind the selected point.

 

3) Insert probe arm between dual tyres of the vehicle to a distance of about 1.2m. lining up arm by eye in such a position that rubbing of probe arm and tyre walls does not occur.

 

4) While truck is standing, record initial reading of dial. Turn on vibrator buzzer before taking first reading.

 

5) Vehicle should be moved slowly (2 Kmph) and smoothly forward to at least 3m past the tip of the beam. The beam operator should catch to see that the probe arm does not rub. The maximum dial reading will occur when wheels are opposite the contact point,

record this value.

 

6) After a reasonable length of time or when the dial needle has come to rest final reading should be recorded.

 

Calculations:

 

1) The maximum deflection is the difference between the initial and maximum readings multiplied by two.

2) The rebound deflection is the difference between the second reading and the final reading multiplied by two.

 

 

3) The residual deflection is the difference between the initial reading and final reading multiplied by two.

 

CBR Curve for Flexible Pavement Design:

 



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