CONTENT
| SL No | TYPE OF INSPECTION | |
| 1 | DETERMINATION OF SURFACE IRREGULARITIES (UNEVENNESS) | |
| 2 | DETERMINATION OF SURFACE UNEVENNESS BY AUTOMATIC ROADUNEVENNESS RECORDER / ROUGHOMETER. | |
| 3 | DETERMINATION 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:
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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