IRC: 76-1979

TENTATIVE GUIDELINES
FOR
STRUCTURAL STRENGTH
EVALUATION
OF
RIGID AIRFIELD PAVEMENTS

THE INDIAN ROADS CONGRESS

 )

MEMBERS OF THE SPECIFICATIONS AND STANDARDS COMMITTEE

J.S. Marya Director General (Road Development) & Addl.
( Chairman ) Secretary to the Government of India, Ministry of
Shipping and Transport
R.P. Sikka Chief Engineer (Roads), Ministry of Shipping and
( Member-Secretary Transport
Qazi Mohd. Afzal Development Commissioner, Jammu & Kashmir
R.C. Arora N.D.S.E. Part I, New Delhi
R.T. Atre Chief Engineer & Joint Secretary, Maharashtra, PW &
H Department
6. M.K. Chatterjee Chief Executive Officer, West Bengal Industrial Infra-
Structure Development Corporation
7. E.C. Chandrasekharan Chief Engineer, Pamban Bridge Project, Madras
8. M.G. Dandavate Engineer, Concrete Association of India
9. J.Datt Chief Engineer (Retd.), Greater Kailash, New Delhi-48
10. Dr. M.P. Dhir Deputy Director, Head, Roads Division, Central Road
Research Institute
11. Dr. R.K.Ghosh Deputy Director, Head, Rigid & Semi Rigid Pavements
Division, Central Road Research Institute
12. B.R. Govind Director of Designs, Engineer-in-Chief Branch, AHQ
13. I.C. Gupta Engineer-in-Chief, Haryana P.W.D., B & R
14. S.A. Hoda Project Manager-cum-Managing Director, Bihar State
Bridge Construction Corporation Ltd.
15. M.B. Jayawant Synthetic Asphalts, 13, Kantwadi Road, Bombay-400050
16. D.R. Kohli Manager, Electronics Data Processing, Bharat Petro-
leum Corporation Ltd.
17. S.B. Kulkarni Manager (Asphalt), Indian Oil Corporation Ltd.
18. P.K. Lauria Addl. Chief Engineer, P.W.D. (B. R.), Rajasthan
19. H.C. Malhotra Engineer-in-Chief and Secretary to the Government of
Himachal Pradesh, PWD.
20. M.R. Malya Development Manager, Gammon India Ltd., Bombay
21. O. Muthachen Poomkavil House, P.O. Punalur, Kerala
22. K. Sunder Naik Administrator, U.K. Project, Gulbarga-585102
23. K.K. Nambiar "Ramanalaya", 11, First Crescent Park Road, Gandhi-
nagar, Adyar, Madras-600020
24. T.K. Natarajan Deputy Director, Head, Soil Mechanics Division, Central
Road Research Institute
25. M.D. Patel Secretary to the Govt, of Gujarat, Buildings & Com-
munication Deptt.
26. Satish Prasad Manager, Indian Oil Corporation Ltd.
27. S.K. Samaddar Chief Project Administrator, Hooghly River Bridge
Commissioners, Calcutta-700021
28. Dr. O.S. Sahgal Principal, Punjab Engineering College, Chandigarh
29. N. Sen Chief Engineer (Retd.), 12-A, Chitranjan Park, New
Delhi
30. S.N. Sinha 49-B, Sri Krishna Puri, Patna
31. D. Ajitha Simha Director (Civil Engineering), Indian Standards Institution
32. Maj. Genl. J.S. Soin Director General, Border Roads
33. Dr. N.S. Srinivasan Chief Executive, National Traffic Planning & Automa-
tion Centre, Trivandrum
34. Dr. Bh. Subbaraju Sri Ramapuram, Bhimavaram —
534202 (A. P.)
35. Prof. C.G. Swaminathan Director, Central Road Research Institute
36. Miss P.K. Thressia Chief Engineer (Construction), Trivandrum, Kerala
37. The Director Highways Research Station, Madras
(Prof. G.M. Andavan)
 IRC : 76-1979

TENTATIVE GUIDELINES
FOR
STRUCTURAL STRENGTH
EVALUATION
OF
RIGID AIRFIELD PAVEMENTS

Published by

THE INDIAN ROADS CONGRESS

Jamnagar House, Shahjahan Road,
New Delhi— 110011

Price Rs.80/-
(F (Plus Packing & Postage )
IRC : 76-1979

First Published: April 1980

(Rights of Publication and Translation are reserved)

Printed at PRINTAID, New Delhi

 IRC : 76-1979

TENTATIVE GUIDELINES FOR STRUCTURAL STRENGTH

EVALUATION OF RIGID AIRFIELD PAVEMENTS

1. INTRODUCTION
1.1. Pavement evaluation, in its most common connotation,
implies the assessment of residual or available structural strength of
the pavement. Evaluation is normally required either in connection
with checking the adequacy of the existing pavements for increased
design loads, or working out suitable overlay designs to restore or
enhance their structural capacity. Evaluation is also needed to
check the quality of a new construction.

1.2. As regards structural evaluation of rigid pavements while

both direct load test and indirect reverse design methods are in use
in the country, no guidelines about the criteria and methods of
evaluation have so far been laid down. It is with a view to provid-
ing a standard basis for evaluation of the rigid airfield pavements,
both in respect of test procedures and the evaluation criteria, that
these guidelines have been prepared. Suggestions are also included
for simplifying the test procedures where it is felt that this could be
done without sacrifice of accuracy.

These guidelines were approved by the Cement Concrete Road

Surfacing Committee (personnel given below) in their meeting held
at Hyderabad on the 5th January, 1976.

K.K. Nambiar ...Convenor

Dr. R.K. Ghosh ...Member-Secretary

MEMBERS
D.C. Chaturvedi K.C. Mital
Dr. M.P. Dhir C.V. Padmanabhan
M.G. Dandavate N.L. Patel
P,J. Jagus K. Krishna Mohan Rao
M.D. Kale P.S. Sandhawalia
Brig. R.K. Kalra Brig. Gobinder Singh
D.N. Khurana S.B.P. Sinha
Dr. S.K. Khanna N. Sivaguru
Y.K. Mehta Dr. H.C. Visvesvaraya

Director General (Road Development) & Addl. Secretary to the

Govt, of India — Ex-Officio
!
IRC : 76-1979

These were then processed by the Specifications and Standards

Committee in their meeting held at New Delhi on the 16th May,
1977 subject to certain modifications which on the authorisation of
the Committee were carried out by Dr. R.K. Ghosh, R.P. Sikka,
B.R. Govind and Lt. Col. Avtar Singh, assisted by Y.R. Phull, M.
Dinakaran and K. Arunachalam. These were later approved by the
Executive Committee through circulation and the Council in their
meeting held on the 28th October, 1979.

2. SCOPE
2.1. The standard describes
the procedure for structural
evaluation of rigid airfield pavements by two alternative methods,
namely the "direct load test method" and the "indirect reverse
design method" which are commonly adopted in the country.

2.2. The "direct load test method" provides actual load

carrying capacity of the pavement taking into account the inter-
action between various pavement layers, extent of load transfer at
joints etc., as actually obtained at site. The "reverse design method"
on the other hand, involves indirect computation of the pavement'
strength based on evaluation of the individual design parameters.
Since the interaction between different parts of the in-service pave-
ment cannot be taken into account in the case of indirect method,
and as the inherent approximations in the method also affect the
evaluated strength value, from the point of accuracy of evaluation
the direct load test method has a definite edge over the indirect
method and is as such to be preferred.

2.3. The actual choice of the method in individual cases will,

however, be dictated by other considerations as well, for instance
the of suitable load testing reaction frame, testing
availability
facilities,time available for the study, the period for which airfield
can be closed to traffic etc. For important works, every effort should
be made to arrange direct load tests. However, when facilities for
this are not available, a reasonably approximate assessment of the
pavement structural strength can be had from the indirect method.
Even if the reverse design method has to be adopted, it would be
worthwhile conducting a few direct load tests for comparison. These
tests could be done at locations where indirect tests are proposed to
be carried out, so that the results could be tied together to assess
actual interaction of the pavement components obtaining at site.

2.4. Both in the case of direct and indirect methods, after

fielddata have been collected, the structural capacity of airfield
pavements can be noted either in terms of LCN (Load Classification

2
 IRC : 764979

Number) or LCG (Load Classification Group) method.

While the
former is based on corner loading, the internal loading
latter is for
conditions. Joints in airfield pavements in the country are in most
cases not provided with dowel bars. For these situations, the corner
loading condition is more appropriate. As such the LCN
rating
system is considered more suitable for conditions existing in the
country, and the procedures described in the guidelines are based on
that system.

2.5. The procedures recommended are intended for rigid air-

field pavements in sound structural condition. If the pavement has
any localised defective spots, such locations should not be selected
for overall structural evaluation, as this might result in gross under
rating of the pavement. Such defective spots should be grouped to-
gether and separately investigated to evaluate the reasons for the
defects, as remedy for these may be in measures like mud jacking,
improvement of drainage, removal and replacement etc., and not in
superimposition of a thick overlay which may provide only temporary
relief.

3. DIRECT LOAD TEST METHOD

3.1. General

The direct load test method essentially involves the application

of static loads through a rigid plate on the existing pavement and
noting the response in the form of deflections/strains. Based on the
data collected, the structural capacity of the pavement is rated in
terms of LCN through the use of appropriate charts. This method
has the advantage that it gives the actual load carrying capacity of
the pavement at the time of test, taking the different interactions into
account. Since the results obtained by this method are dependent
on test conditions, such as the time of testing, location of test points
etc, it is necessary to follow a common procedure to determine the
critical load carrying capacity of the pavement. Guidelines with
t

regard to these conditions are given below:

(1) Period of testing: The period of testing should be such

that it is critical for foundation strength (i.e. when the foundation
saturation is at maximum or near-maximum) as well as for load
transfer especially in the case of dummy joints (i.e. when the mean
pavement temperature is minimum during the year). Early winter,
following the rainy season, can be a good working compromise from
these considerations. If it becomes necessary from other considera-
tions to conduct the tests at any other period, appropriate adjust-
ments, based on actual tests or engineering judgement, should be

3
IRC : 76-1979

applied in respect of foundation strength and degree of load

transfer.

(2) Locations of test: In the case of undowelled pavements,

50 per cent tests should be carried out at the junction of transverse
expansion joint and longitudinal construction joint, and the remain-
ing at free slab corner at the junction of transverse and longitudinal
expansion joint. For pavements having dowelled transverse ex-
pansion joints, however, the junction of longitudinal expansion or-
construction joint and the transverse dummy joint may be chosen as
the test locations.

(3) Number of test locations and selection of test sites: For

airfield evaluation one test is recommended for every 60 m
length in
case of runways and 60-90 m
length in case of taxi tracks and
aprons. In general, a total of 15-20 determinations may be
required for proper statistical evaluation of the test data. These
recommendations may be treated as general broad guidelines only,
and depending on the site conditions the test locations and frequ-
ency may be left to the discretion of the Engineer-in-Charge.

(4) Time of testing: Since tests are conducted on slab corners

which are the most vulnerable portions of the slab, as far as possible
the tests should be done at the most critical time of the day when
the load carrying capacity of the corners will be the minimum from
considerations of pavement warping, viz. early hours in the morn-
ing.

3.2. Test Procedure

3.2.1, General: The test is performed on selected pavement

corner by loading it through 45 cm diameter plate and measuring
deflections at the top of the loaded slab corner and the three adjoin-
ing slab corners. The loading frame for use in the tests should be of
a capacity 50 per cent higher than the equivalent single wheel load
corresponding toihe original design LCN value of the pavement.

A thin layer of fine sand or plaster of paris may be used

below the test plate, where required, to ensure fulT contact with the
pavement. Also a seating load of 3000 kg may be applied initially
for about 10 seconds and released before commencement of the test
proper.

Basically two procedures (I and II) are available for the test
within the scope of LCN method viz. load test well beyond cracking
and load test upto imminent cracking. A third alternative, that is
testing upto standardised value of working deflection to directly

4
 IRC : 76-1979

obtain the working load, also suggested as a modified and simpler

is

procedure (III) requiring much less test loads as compared to the

first two alternatives. While any of the first two procedures may be
adopted independently for the test, the third procedure based on
working load deflection may be adopted in conjunction with either
of the first two procedures. This procedure will be specially useful
where either the testing is to be done expeditiously or where crack-
ing of slabs due to testing is to be avoided.

3.2.2. Procedure I. Load test beyond cracking: In this pro-

cedure, in addition to the deflection gauges on slab corners, four
additional deflection gauges are located along the bisector of the
loaded corner angle as shown in Fig. 1. Load is applied in equal
increments of 3000 kg and deflection readings taken after each load
increment. After occurrence of cracks, the gauges on the main
slab beyond the corner crack will register a lower or no increase in
deflection with increase in load, while in case of other gauges includ-
ing gauge 1 (Fig. 1) there will be a sudden increase. As soon as
IRC : 76-1979

this change in deflections is noticed, the test is stopped, and the

load corresponding to the point of change taken as the failure load.
As per the standard LCN procedure, safe working load is obtained
by applying a factor of safety of 1.5 for non-channelised traffic
areas and 1.8 for channelised traffic areas.

3.2.3. Procedure II. Load test upto imminent cracking: In

thisprocedure, instead of deflection gauges four mechanical strain
gauges (Fig. 2) are used along the bisector of the loaded corner to

Fig. 2 Arrangement of strain gauges for detection of crack

incidence for corner load test in airfields

detect the imminence of cracking, so that the test load may be taken
as close as possible to the point of failure without actually causing
a crack. As soon as the reading on any one strain gauge starts
increasing rapidly relative to the adjacent gauges, the corresponding
load is taken as the failure load and the safe working load obtained
by applying a factor of safety of 1.5/1.8 as in procedure I.

In case of both the above procedures I and II, if the pavement

does not show any sign of cracking upto the full capacity of the

6
 IRC : 76-1979

loading frame, the safe working load can be calculated from the
maximum applied load.

3.2.4. Procedure III. Load test upto a standard working

deflection: In this procedure, a working deflection value is determi-
ned for each specific case by conducing failure load tests at 3-4
locations only and noting the failure deflection. The working
deflection is obtained by applying a factor of safety of 1.5/1.8 to the
average of the observed failure deflections.

This procedure is based on the fact that the load deflection

curves for such pavement tests are practically linear within the
failure load limit, and hence it is possible to apply factor of safety
of 1.5/1.8 to faiiure deflection instead of the failure load. The load
corresponding to the safe working deflection is taken as the safe
working load. Since the value of failure deflection for a pavement
is affected by factors such as slab thickness, concrete strength, area
of loading, subgrade stiffness, etc., it is not possible to stipulate a
single value of failure deflection. Because of this, it is necessary
that in every case atleast 3-4 tests must be carried out to determine
the failure deflection as the basis for further evaluation.

This procedure not only ensures that the pavement does not
get cracked, but also smaller loads and less time are needed for
completion of the test. Only four deflection gauges at the four
corner tips are required for the test and no additional deflection or
strain gauges are needed. In this deflection-based procedure,
observations should be taken at equal deflection increments of
0.15-0.25 mm
to obtain atleast 5-6 readings within the recommended
working deflection range.

Note : The possibility of an occasional erratic result cannot however be

ruled out, due to factors such as subgrade pumping, poor local drai-
nage unusually weak spots in foundation or the slab etc. In such
cases, when the slab cracks during the test, the working load should
be obtained as per procedure I by applying a factor of safety of
1.5/1.8 to the failure load.

3.2.5. Illustrative load deflection/strain curves for the three

procedures are shown in Fig. 3.
§

3.3. Adjustment for Load Transfer

3.3.1. The load carrying capacity of a slab evaluated by the

direct load test method includes a component due to load transfer
at the joints. This component will always be available if the test
is carried out at the minimum temperature the slab is likely to

7
IRC : 76-1979

8
 —

IRC : 76-1979

attain. As such
if the load test is conducted when the slab is at its
minimum temperature, no correction for load transfer need be ap-
plied. However, if the test is conducted when the slab temperature
is higher than the minimum attainable in service, the evaluated load

capacity should be reduced by a suitable correction factor to take

care of the possible reduction in load transfer when the slab tempe-
rature falls down to the minimum value.

3.3.2. According to the current state of knowledge, adjust-

ment for load transfer is an arbitrary process based on experience.
It will be, therefore, desirable to conduct the load tests during the
coldest part of the year as far as feasible so that the need for load
transfer adjustment is obviated. Where this is not possible, the
correction factor to be applied may be evaluated on the following
lines. The load transfer actually available during the time of test
determined. The load transfer which will always be available
is first

even when the slab is at its minimum temperature is then assessed.

The correction factor is taken to be the difference between the two.

3.3.3. The load transfer actually available during time of the

test can be evaluated by assuming that its magnitude is directly
proportional to the measured deflection of the respective corners.
If SI, S2, S3 and S4 are the recorded deflections of gauges 1,2,3
and 4 respectively (see Fig. 1 for position of the gauges), the load
transfer will work out to:

SI
(1 ~ si + s2+^ ~Tsr ) x 10 °P ercent

3.3.4. The minimum lead transfer which be will always

available is assumed as the average of the lowest of the quartile
observed load transfers subject to a maximum value of 20 per cent
(see foot note* for procedure). If the measured load transfer is x
per cent and the minimum load transfer y per cent, the corrected
load capacity will be [ 100-(x y)] per cent of the measured load
capacity.

3.3.5. The above applies to cases where the joints are not
provided with load transfer devices. If the pavement contains load
transfer devices like dowels or continuous reinforcement, the com-
ponent ofload transfer will be higher and most of it will be available

*Note : A frequency distribution table is prepared for the measured load

transfers. The lower values below the first quartile (25 per cent
frequency) are considered and their average value obtained subject
to a maximum of 20 per cent.

9
IRC : 76-1979

even at the minimum temperature. Some reduction should however

be anticipated, to account for which it is recommended that a re-
duction of 10 per cent in the measured load transfer of slab may
be applied.

3.4. Determination of Safe LCN Rating

3.4.1. Knowing the safe working load and the contact area
of the test plate, the LCN rating for pavement of any test location
may be obtained from the standard LCN chart vide Fig. 4.

EQUIVALENT SINGLE WHEEL LOAD OR WORKING LOAD OBTAINED FROM LOAD TEST THOUSENDS OF kg

Fig. 4 LCN in terms of load, contact pressure/diameter

of contact area for rigid pavements

3.4.2. For assessing the safe LCN rating of the pavement as

a whole, the safe working load should be calculated statistically for
a confidence level of 1 in 15, by subtracting 1.5 times the standard
deviation from the average of the working loads calculated with
respect to individual tests.

This value should then be examined in relation to individual

LCN test values to see whether any test locations have exceptionally
low values, so that the same may be considered separately for ap-
propriate remedial measures.

3.4.3. An illustrative example of calculation of safe LCN for

an airfield runway is given in Appendix-1.

10
 IRC : 76-1979

4. INDIRECT REVERSE DESIGN METHOD

4. 1 . General

Where direct evaluation by load tests is not possible due to

any reason, indirect evaluation may be done by reverse design
method.

This method requires the same basic information for evalua-

ting pavement structural strength, as is required in the case of
design for a new pavement (with the obvious exception of design
wheel load, and addition of actual pavement slab thickness), viz:

(i) Flexural strength of concrete in the pavement

(ii) Foundation strength in terms of k-value
(iii) Maximum temperature differential over pavement depth.

It is,however, necessary to determine the actual strength of con-

crete and pavement foundation by appropriate tests. Selection of
test locations for this purpose should be on the same lines as sugges-
ted in para 3.1.

However, if effects of fatigue are also to be evaluated, a few

additional test locations may be selected along the outer edges which
are subject to much less traffic intensity.

*
4.2. Test Procedure

4.2.1. Concrete strength: Either beam or core samples may be

recovered from the test locations for determination of concrete stren-
gth. While it is preferable to have beam samples for direct
determination of flexural strength, core samples may have to be
resorted to many a time from considerations of expediency, available
equipment, time available for investigation, or the need to keep
damage to the pavement from sample recovery to the minimum.

Concrete samples, whether beam or core, should be carefully

examined for quality of compaction, and their dimensions accura-
tely measured. Beam samples may be tested in flexure using third
point loading, and concrete flexural strength determined there from.
For cores, crushing strength results should be corrected for h/d
ratio before determination of corresponding cube compressive
strength. The crushing strength of cylinders with h/d ratio between
1 and 2 may be corrected to correspond to standard h/d ratio of 2
by multiplying with the correction factor obtained from the following
equation:

11
IRC : 76-1979

/=0.11 « + 0.78
Where /= correction factor
n—h\d ratio

The diameter "d" of cores recovered should not be

less than
10 cm concrete with maximum aggregate size of 10
for mm
and not less than 15 cm for concrete with maximum aggregate size
of 40 mm.

Cube compressive strength may be taken as 1.25 times the

corrected cylinder crushing strength. Conversion from cube com-
pressive to flexural strength may be done using the chart given in
Fig. 5.

7 63 25.8

400

300

200

100
10 20 30 40 50 60

2
FLEXURAL STRENGTH - Kg/cm ^X
Fig 5 Statistical correlation between compressive
and flexural strength of concrete

The strength tests should be done in accordance with the

relevant I.S. specifications.

12
 IRC : 76-1979

4.2.2. Foundation strength: Foundation strength in terms of

its A>value is normally determined directly by conducting plate
bearing tests on the foundation on which the slab rests (which could
be earth subgrade, or a sub-base if provided) after removing sections
of pavement slab. The test could be conducted on a 30 cm dia.
Plate and converted subsequently to the standard /c-value for
75 cm diameter plate by the approximate correlation:

& 75 =0.5 k 3o
where & 75 and k 30 are the ^-values for 75 cm and 30 cm diameter
plates respectively. It should, however, be noted that this correla-
tion is based on homogenous foundation conditions, and in the case
of layered construction i.e. when the test is conducted on a sub-
base, the smaller plate will give a greater weightage to the stronger
top layer. In such cases, direct conversion to 75 cm plate value by
the above correlation somewhat over-estimates the foundation
strength and should be regarded as very approximate only.

After conducting the test, the sub-base may be removed upto

the subgrade level, noting its type and thickness, and a plate bear-
ing test conducted to determine subgrade /c-value, if considered
necessary.

If direct determination of foundation /c-value is not possible,

an approximate indirect assessment can be made by conducting in-
situ CBR test on the subgrade. From this, an approximate idea of
the subgrade fc-value may be obtained using the CBR fc-value
correlation given in Table 1.

If any sub-base
is present over the subgrade, due allowance
should be madeincrease in the foundation Zr-value.
for For this
purpose, the charts given in Fig. 6 may be made use of.

Table 1. Approximate k-values Corresponding to CBR Values

for Homogenous Soil Subgrades

CBR value (%) 2 3 4 5 7 10 20 50 100

k-value (kg/cm 8 ) 2.08 2.77 3.46 4.16 4.86 5.54 6.92 13.85 22.16

4.2.3. Supplementary soil tests: Soil plasticity and grain-size

analysis tests should also be carried out for determining the soil
classification of the subgrade.

13
IRC : 76-1979

ujo/^ujo/M *3SV99nS JO dOX NO X

uid^uo/6)! *3SV98nS JO dOl NO M

14
 IRC : 76-1979

4.2.4. Slab thickness: Slab thickness may be determined by

direct measurement of height on core or beam samples recovered
from the pavement slab for concrete strength determination.

4.3. Number of Test Locations and Selection of Test Sites

Same considerations will apply in this case as for the Direct

Load Test Method. Provisions of para 3.1 should therefore be
followed in this respect.

4.4. Analysis of Concrete/Foundation Strength Test Data

The
distribution of test data should be studied carefully both
for concrete and foundation strength to see if the pavement section
under investigation could be divided into zones of distinctly different
concrete and foundation strengths, in which case "typical" strength
values should be separately worked out for each zone. In case
such sub-division is not possible, the strength data may be examined
on an overall basis.

It suggested that the "typical" strength values should be

is

arrived at from actual test data for ensuring a confidence level of

1 in 15. This can be worked out by reducing the average of all the
strength test values by 1.5 times their standard deviation. The
dasign strength values may thereafter be obtained by applying a
factor of safety of -1.1 to the typical values..

4.5. Determination of Pavement Structural Strength

4.5.1. Pavement design procedure to be adopted: The LCN

method for airfield pavement design may be used. Knowing pave-
ment thickness, concrete flexural strength and foundation &-value,
the rated LCN for the pavement can be read directly from the chart,
Fig. 7.

4.5.2. Allowance for load transfer: As in the case of evalution

by direct load test (see para 3.3), it is necessary to assess the mini-
mum load transfer that will be available at any location in the
pavement. The critical locations from this consideration would be
the same as those recommended in para 3.1;

Contribution to load capacity due to load transfer may be

assessed by conducting actual load tests, noting deflections of
adjacent slabs upto anticipated design equivalent single wheel load
for the pavement. Only a limited number of tests on typical joints
would be adequate. (Similar tests can also be conducted to assess
the load transfer capacity of typical cracks in the pavement). The

15
IRC : 76-1979
 IRC: 76-1979

test willhowever, have to be conducted in the coldest part of the

year when load transfer is at its critical minimum; otherwise a
correction factor will have to be applied as explained in para 3.3.

The total assessed load carrying capacity of the pavement at

any location shall be obtained by combining the individual slab load
carrying capacity, as calculated in para 4.5.1., with the load transfer
capacity at joints.

4.5.3. An example illustrating the structural strength evalua-

tion of an airfield pavement by reverse design method is given in
Appendix-2.

17
 IRC : 76-1979

ILLUSTRATIVE EXAMPLE FOR CALCULATION OF SAFE LCN FOR AN AIRFIELD RUNVvl

MEASURED LOAD TRANSFER VALUES

Corrected
Deductable
Failure load/ Safe loads A .1 1 11 s te d load transfer LCN
max. applied Col.(2)/1.5 load transfer p ce
<£''
*CoM3) l (x-X)' Calculations for LCN
load (tonnes) (tonnes) coi. Fig. 4)
Col. 5

-4W
.•nlio limit
I
>

:led safe load

r a tolerance

The corresponding
"
safe LCN for
(from Fig. 4)

Ix=6I1.5
Average x=20.4 1=95.23

*Nole : Examining the individual LCN values vis-a-\is L.C.L. (lower control limit I. it is seen that only 2 values marked with aslertcks fall below
L.C.L. out of a total of 31) s.ilucs .vorresponding to tolei.ime kvclof.. in 15, However, in Mew of relalivcl> low value vis-a-vis overall
1

safe LCN rating of 54. ihe location with LCN 4(, may he investigated separately to ascertain if there is any specific reason for its low
rating, and foi leeiitieaiion of the same.

19
 /

: 76-1979

Appendix 2

AN EXAMPLE ILLUSTRATING THE STRUCTURAL STRENGTH

EVALUATION OF AIRFIELD PAVEMENT BY REVERSE
DESIGN METHOD

Test Data

Slab thickness, //=25 cm

Compressive strength (kg/cm 2 ) of pavement concrete cores, 25 cm high
15 cm dia (at 10 locations)
385, 355, 320, 360, 340, 295, 325, 360, 330, 310.

CBR values of subgrade (at 10 locations)=13.0, 12.0, 10.5,

10.0, 11.0, 12.5, 11.5, 12.5, 10.5, 11.5.

Thickness of WBM subbase=25 cm

Measured load transfer, per cent
(at 10 locations)^ 12.66, 13.03, 8.40, 14.63, 6.80, 10.1,
10.1, 8.9, 6.7, 8.7

Tyre pressure of predominant gear assembly using the

pavement = 10 kg/cm 2

Calculation of Structural Strength of tire Pavement

(1) Assessment of Flexural Strength of Concrete

From the 10 values of core compressive strength, average, x=338 kg/cm 2

Standard deviation, a = 27.3 kg/cm 2

.-. for a tolerance level of 1 in 15,

min. core strength =x — 1.5 a = 297 kg/cm 2
Applying a factor of safety of 1.1,
design core strength=297/l. 1=270 kg/cm 2

25
Height/diameter ratio of cores, n=-r^ -=5/3

Correction factor for height/diameter ratio :

/=0.11 k+0.78
=0.1833 + 0.78=0.9633
Corrected core compressive strength
=270x0.9633=260 kg/cm 2
Cube Compressive Strength
= 1.25 x core compressive strength
= 1.25x260=325 kg/cm 2

From Fig. 5, for compressive strength of 325 kg/cm 2 , flexural strength of

pavement concrete & =40 kg/cm 2
.

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 IRC : 76-1979

(2) Assessment of k-value of sub-base

From the 10 values of subgrade CBR,

average, x=11.5 per cent
Standard deviation, a=1.0 per cent
For a tolerance level of 1 in 15,
min. CBR value=x-1.5 ©=11, 5-1.5=10.0 per cent
From Table 1, k-value of subgrade,
(corresponding to CBR value of 10%) =5.54 kg/cm /cm
2

From Fig. 6(a),subgrade k-value of 5.54 kg/cm 2 /cm and subbase

for
thickness of 25 cm, k-value on top of subbase=8 kg/cm
3
.

(3) Assessment of Pavement Slab LCN

From Fig. 7, for unchannelised traffic (for middle of runway),
2
for 6=25 cm, /c=8.0 kg/cm and/&=40 kg/cm
2
,

Pavement LCN=40
3. Correct ion for Load Transfer

(1) Calculation of Minimum Load Transfer

all the values obtained are less than 20 per cent, and none of them
is
Since
zero, taking the average of all these values, the level to which the higher
values of load transfer will get reduced = 10.0 per cent.
.-. adjusted load transfer values (per cent) = 10.0, 10.0, 8.40, 10.0, 6.8, 10.0,

10.0, 8.9, 6.7, 8.7

From these values, x=8.95%, a = 1.32%

Most probable min. available load transfer

=2-1.5 a=8.95-1.5xl.92
= 8.95-1.98=6.97%
(2) Modified Pavement Slab LCN taking into account Load Transfer

From Fig. 4, for tyre pressure of 10 kg/cm 2 ,

equivalent single wheel load
(ESWL) corresponding to LCN 40 (without load transfer) = 13000 kg. .

Accounting for load transfer,

Actual load carrying capacity of

the pavement = 1 3,000 x =13980 kg.

9?

Corresponding LCN (from Fig. 4) =42.

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