Highway Pavements: Design, Types, Flexible and Rigid Pavement, notes and Functions . Also Learn about:- 1. Design of Highway Pavements 2. Pavement Evaluation 3. Strengthening.
Design of Highway Pavements:
Pavement design is governed by a number of considerations/factors, as given below:
‘Design period’ or ‘design life’ of a pavement is the period of time for which the initially designed pavement will last and serve the traffic needs without needing any major rehabilitation or repair. If the level of serviceability is the criterion, the design life is the period in which the pavement deteriorates from its initial level of serviceability to its final level or minimum acceptable level of serviceability. This design period is dependent on the nature and level of maintenance schedule.
The design life may be taken as 10 to 20 years for low-traffic volumes (flexible pavements) and 20 to 30 years for high-traffic volumes, (rigid pavement) depending on the designer’s discretion [IRC37-2001 and IRC58-2002].
The following are the traffic-related factors that govern the design of the pavement:
(a) Traffic volume
(b) Wheel load, axle load, and wheel configuration
(c) Standard wheel load and standard axle load.
(d) Tyre pressure and contact pressure
(e) Equivalent single wheel load
(f) Load equivalency factor
(g) Vehicle damage factor
(h) Load safety factor.
The primary climate- and environment-related factors which affect the design and performance of pavements are:
i. Rainfall and Drainage:
The intensity and duration of rainfall causing surface runoff and percolation into the ground recharging the ground water affect the stability of pavements by softening the subgrade and the base and sub-base courses. Variations in the ground water table may cause shrinkage and swelling of fine-grained soils, and loss of supporting power of the subgrade. Capillary rise also may cause damage. Adequate drainage of subgrade and pavement layers is therefore very important for good performance of pavements.
Under low temperature conditions, water in the soil voids freezes and tends to cause frost heave underneath the pavement. Also, under rising temperatures when water begins to thaw, it causes loss of the bearing power of the subgrade. Frost action is possible with finegrained soils when the settings are not favourable for proper drainage in cold regions. Thus, frost action on pavements can be overcome through adequate provisions for drainage of water.
Seasonal variations of atmospheric temperature-short-term and long-term-between day and night and between summer and winter can affect the performance of a pavement.
Flexible pavements – A thick bituminous course can get affected by the temperature gradients between the bottom and top and result in cracking and loss of stiffness.
Rigid pavements – Cement concrete pavements can get affected by temperature gradients between the top and bottom faces. The phenomenon of ‘warping’ causes temperature stresses in the body of the pavements, which combines with the stresses caused by traffic loads. These combined stresses have to be within the limits of allowable values.
The annual average pavement temperature is taken as 35°C (IRC: 37-2002 and IRC: 81-1997). The surface deflections measured in a Benkelman beam test are corrected to this standard value if the temperature is different from 35°C.
The importance of subgrade strength in the design of flexible pavements cannot be underestimated. As any deformation of the subgrade automatically gets transmitted to the surface through the overlying courses, the bearing power of the subgrade has to be ensured so that subgrade failure does not lead to the failure of flexible pavements including bituminous surfaces.
Even in the case of rigid pavements, a sufficiently strong subgrade helps to increase the durability and trouble-free performance.
One important factor in retaining the strength of the subgrade is proper drainage arrangements-surface and sub-surface.
Pavement material consists of soil or earth, especially subgrade soil, fine aggregate, granular coarse aggregates, binders like bitumen, tar, lime, cement, and other miscellaneous materials. Engineering properties of these materials play an important role in the design of thickness of the pavements and the course underlying them. In the case of rigid pavements, the modulus of subgrade reaction of soil is important.
The concept of ‘reliability’ in pavement design, introduced by the AASHTO (1993), is relatively recent. This is considered to account for variations in the design inputs relating to soil characteristics, traffic factors, climate factors, construction quality, and so on.
‘Reliability’ means the ‘probability’ that any particular damage or distress of the pavement will remain within the permissible level during its design life. This automatically incorporates a certain amount of ‘certainty’ that the pavement will give satisfactory service during the entire design period. ‘Reliability’ is expressed as percentage.
Based on AASHTO guidelines, the reliability values for flexible pavements in India may be taken as given below:
The performance prediction error or overall standard deviation may be taken as 0.4 to 0.5 for flexible pavements and 0.3 to 0.4 for rigid pavements in the above reliability values.
The ‘life-cycle cost’ of a highway is the overall cost including initial cost, maintenance cost and vehicle operations cost. This is to be properly evaluated for any highway project during the planning stage itself. Naturally, it has to be considered in the design stage too-with the choice of pavements type, availability of materials, traffic factors, and the importance of the project. Choice of appropriate construction techniques is important. Stage-construction technique may be employed if paucity of funds is a constraint.
Pavement evaluation may be defined as a technique of assessing the condition of a pavement from the point of view of structural adequacy and surface characteristics.
Such assessment serves a variety of functions:
1. Assessing the maintenance needs of a pavement
2. Assessing the need for provision of an overlay on damaged pavements
3. Studying the long-term performance of different specifications and methods of construction of pavements for evolving better techniques
Structural adequacy evaluation may be done by conducting field tests.
(a) Visual rating
(b) Present serviceability index approach
(c) Roughness measurements approach
(d) Non-destructive testing of pavement deflection
This is a simple method of inspecting a pavement surface for assessing the magnitude and severity of various types of damage – rutting, ravelling, patching, corrugations, alligator cracking, longitudinal cracking, transverse cracking, and failure. A widely used method, known as deduct value or deduct point method, was developed by Texas A&M University in 1974.
Certain deduct points are associated with certain types of distress based on their extent, upon visual inspection. The sum of all such points deducted from a perfect score of 100 gives the overall rating of the pavement. However, in view of the subjective element, this method is no longer popular.
A rating system involving the measurement of permanent deformation, riding quality, the extent of cracking and patching of a pavement had been developed by the AASHTO Equations for PSI were developed for flexible pavements as well as rigid pavements. These equations, involve slope variance – an index of the longitudinal profile (SV), rut depth (RD), cracking distress (C), and patched area (P). In the case of rigid pavements, rut depth (RD) is not applicable.
The longitudinal profile is monitored by a profilometer, developed by the AASHTO.
(c) Roughness Measurements Approach:
The riding quality of a pavement is governed mostly by its structural adequacy, in addition to the traffic load repetitions, the specification used for the construction, and maintenance inputs, if any, play a role in affecting the quality of a ride. Thus, a measure of the pavement performance can be its roughness.
Roughness may be measured by direct measurement of the longitudinal profile or by using response-type instruments; the former may be done by direct levelling, which is accurate, but tedious, or it may be by using a device called profilometer.
A typical instrument of the response-type is the towed fifth wheel bump integrator, developed in the U.K.
This is a popular roughness measuring device, developed and widely used in the UK. A similar device, indigenously produced, is used in India. It is showed schematically in Fig. 10.5.
This consists of a trailer supported on a single wheel mounted inside a rectangular frame of steel. The wheel is a pneumatic tyre of standard size and supports the chassis of the frame by means of two leaf springs. The frame has a tongue at the front end with a hitch for connecting to the towing vehicle.
The vertical movement of the frame is dampened by a piston-rod, connected to a dashpot filled with a standard fluid. The vertical movement between the axle and the chassis is recorded by a unit designed for the purpose.
For every 2.5 cm of forward horizontal movement, a pair of contacts is inserted in an electro-magnetic counter and the circuit gets closed. The revolutions of the wheel enable the measurement of the distance travelled by the unit. The electromagnetic counter in the electric circuit records the roughness measurement as well as the distance travelled.
A jeep, fitted with a 12 Volt-battery, is used for towing the vehicle. The standard speed of the unit is 32 km/h for the measurement of the roughness. The roughness measurements are obtained in mm/km.
For accuracy in measurement, the following precautions are necessary:
(i) Checking the tyre size.
(ii) Ensuring correct level of fluid in the dashpot.
(iii) Measuring at the recommended standard speed.
The roughness measurements obtained will form the basis of evaluating the pavement and for planning the necessary maintenance operations.
Pavements deform elastically under traffic loads; the deflection depends on several factors such as wheel load, pavement thickness and quality, subgrade soil, moisture and compaction, drainage facilities, and environmental factors such as temperature.
Different types of pavement deflection testing devices or equipment have been evolved.
Some of them are:
(i) Benkelman beam deflection device
(ii) Lacroix deflectograph
(iii) Dynaflect device
(iii) Falling weight deflectometer
(iv) Dynamic cone penetrometer
Among these, the first one – Benkelman beam deflection device – is the most popular one, and the one recommended by IRC for the design of overlays for strengthening flexible road pavements in India. Hence, the Benkelman beam deflection device is presented here.
This is a handy instrument devised by A.C. Benkelman (1952) to measure the response of a flexible pavement in terms of surface rebound deflections under standard axle loading from a truck. The device was made for the road tests conducted by the Western Association of State Highway Organisation (WASHO).
Several organisations have adopted the Benkelman beam deflection method for structural evaluation of flexible pavements and overlay design – the AASHTO, the Asphalt Institute, the Transport and Road Research Laboratory (TRRL, London), the Canadian Good Roads Association (CGRA), besides the Indian Roads Congress (IRC) to name a few.
The IRC recommendations for this test are given in “IRC: 81-1997, ‘Guidelines for strengthening of flexible road pavements using Benkelman beam deflection technique’, Indian Roads Congress, New Delhi, 1997”. A pictorial view of the device is given in Fig. 10.6.
The Benkelman beam consists of a lever 3.66 m long, pointed at 2.44 m from a probe, which can be set to be in contact with the pavement. The deflection of the pavement surface produced by the test load is transmitted to the other end of the beam where it is measured by means of a dial gauge or a recorder.
The movement at the dial gauge end is one half of that at the probe. (This is because the distance of the probe and the dial gauge from the pivot is in the ratio of 2:1). The test load on the dual wheel can be in the range of 2.7 to 4.1 t.
The procedure given by the IRC for measuring the rebound deflection is given below (IRC: 81-1997):
(i) Select at least ten points along the outer wheel path for each lane (at least 60 cm from the edge of the pavement.)
(ii) Insert probe of the beam between the dual wheels, exactly over the point where the deflection is to be measured. (The standard wheel load is 4085 kg and the tyre pressure is 5.6 kg/cm2).
(iii) The initial reading of the dial gauge is noted (Do).
(iv) The truck is driven forward slowly and the dial gauge readings are noted when the truck stops at 2.7 m and 9 m from the measuring point, and the rate of rebound or recovery equals 0.025 mm per minute or less (D1 and D2, respectively).
(v) The pavement temperature is recorded.
If (D1 – D2) ≤ 0.025 mm, the actual rebound deflection, D1’ is
D = 2(D0 – D2) … (10.1)
But, if (D1 – D2) > 0.025 mm, correction for the vertical movement of the front leg has to be applied.
The corrected or true deflection is given by the formula:
D = 2(D0 – D2) + 2.91Y … (10.2)
Where, Y = Vertical movement of the front legs, i.e., twice the difference between the final and intermediate dial readings.
The residual deflection is twice the difference between the final reading and the initial reading.
The plan view of the Benkelman beam when the probe is inserted in between the dual tyres of a truck rear axle is shown in Fig. 10.7.
Calibration of Benkelman beam: The dial gauge can be calibrated by placing the probe on a metallic plate of known thickness, as shown in Fig. 10.8.
The reading on the dial gauge should be half the thickness of the plate. The reference beam should be horizontal and the ground should be stable and free of vibrations. The probe beam should have free movement at the pivot.
Road inventorying is a systematic procedure of collecting details of the existing condition of a pavement constructed some time ago and in operation.
This serves a variety of purposes for assessment of:
i. Deficiencies in the existing road relating to the riding quality, geometries, drainage structures, etc.
ii. Structural adequacy of the road pavement to determine the need for overlay.
iii. Short- and long-term maintenance needs.
Road inventorying may be done at least once in five-years to cover the following features:
1. Classification and description
3. Information on urban/rural sections
4. Nature of terrain
5. Pavement width
6. Pavement surface
7. Riding quality
9. Horizontal alignment
10. Vertical alignment
12. Cross-drainage works
13. Drainage structures
14. Traffic signs, signals and markings
15. Roadside amenities.
Manual method of inventorying is time-consuming and tedious. Hence, modern practice is to carry out the inventorying with the aid of an instrument car and store the data in the form of data bank in a computer.
Strengthening of existing highway pavements may be required for any of the following reasons:
(i) Deterioration of the pavement under increased wheel loads and load repetitions; when such deterioration cannot be made good by simple maintenance measures like patch repair/periodic renewals, strengthening measures are called for.
(ii) Although the existing surface is in a reasonably good condition and capable of giving satisfactory level of serviceability, the spurt of traffic volume and traffic loads owing to an unexpected economic and industrial development in the area may demand a pavement of greater strength and thickness, and strengthening measures may be needed. This comes more in the category of preventive maintenance.
Under both these situations, strengthening by providing additional thickness of pavement, called an overlay is adopted. A provision of such an overlay overcomes the functional as well as structural inadequacies of the existing facility. Besides estimation of the design traffic, estimation of the strength of the existing pavement and determination of the type and thickness of the overlay are important steps in overlay design.
The design of overlays is mostly empirical, except that field observations of certain serviceability parameters such as pavement deflections and roughness with the aid of equipment devised for the purpose are utilised on a scientific basis to the extent possible.
Overlays are categorised based on the type of existing pavement and that of the overlay:
(a) Flexible overlay over flexible pavements
(b) Rigid overlay over rigid pavements
(c) Flexible overlay over rigid pavements
(d) Rigid overlay over flexible pavements.
(a) Flexible Overlay over Flexible Pavements:
IRC guidelines given in “IRC: 81-1997: ‘Guidelines for strengthening of flexible road pavements using Benkelman beam deflection technique’, Indian Roads Congress, New Delhi, 1997.” The design is based on the deflection values of the pavement surface measured by Benkelman beam.
Before these deflections are used in the design, two corrections have to be applied to the measured values.
(i) Temperature Correction:
Deflections are found to be sensitive to temperature variations. IRC guidelines specify a standard temperature of 35°C for the pavement surface for deflection measurements. A correction of 0.0065 mm per each degree difference with respect to the standard value of 35°C is required. The correction is positive for temperatures lower than the standard and is negative for those higher than the standard.
For high attitude areas, an ambient temperature of 20°C is recommended by IRC, with no corrections.
(ii) Correction for Seasonal Variation of Subgrade Moisture:
Deflections are to be measured when the subgrade is moist, i.e., soon after monsoon. If the observations are made in dry season, a correction factor is to be applied. The correction factor recommended is 2 for clayey subgrade and 1.2 to 1.3 for sandy subgrade.
A large number of deflections (n) are taken along wheel paths (D). The mean, D̅, and the standard deviation, σ, are calculated from the observed corrected values.
However, recent research findings from a research project, sponsored by the Ministry of Surface Transport, suggest that the characteristic deflection may be taken as the mean plus two standard deviations-
Dc = D̅ + 2σ … (10.5)
Allowable deflection values suggested by MOST based on the findings of their research project are given below-
If the characteristic deflection is greater than the allowable deflection, the thickness of granular overlay (WBM) is obtained from the following formula (IRC: 81-1984) –
h = thickness of granular or WBM overlay in mm
Dc = Characteristic deflection
Da = Allowable deflection
R = A constant, whose value is taken as 550
However, the latest IRC guidelines – IRC: 81-1997 – does not include this equation; instead, a set of curves are given for determining the thickness of the overlay from the characteristic deflection for different traffic volumes in msa (Fig.10.10).
If other materials other than bituminous macadam are used, the following conversion factors are recommended –
1 cm of bituminous macadam = 1.5 cm of WBM/WMM/BUSG
= 0.7 cm of DBM/Bituminous concrete.
Overlays for Rigid Pavements:
Rigid pavements or cement concrete pavements can be rehabilitated by providing a rigid or a flexible overlay.
Types of Rigid Overlays:
Rigid overlays are mostly cement concrete slabs.
Three types of rigid overlays are possible:
(i) Bonded or monolithic overlays
(ii) Partially bonded overlays
(iii) Un-bonded overlays
(i) Bonded or Monolithic Overlays:
In this type, a thin overlay slab is laid over the existing slab after preparing the existing surface by etching or scarifying by milling machines and coating it with cement mortar.
Such an overlay acts monolithically with the existing slab and requires the minimum thickness (25 mm). However, this is not adequate if the existing slab has badly failed.
(ii) Partially Bonded Overlays:
In this method, the overlay slab is placed over the existing slab after cleaning the surface. The minimum thickness in this case is 120 mm. This type is also not recommended when the existing slab has severely failed.
(iii) Un-bonded Overlays:
A separation layer or course is laid over the existing slab (this layer is usually of a bituminous material and the overlay slab is laid over it). The action of this overlay is independent of the existing slab. Such overlays can be provided over a badly failed existing pavement also, with a minimum thickness of 150 mm.
Reflection cracks may occur on the bounded overlays, but not on unbounded ones. Steel is not provided in thin overlays. The steel requirement of partially bonded or unbounded overlays is independent of the steel in the existing slab.
Design of Rigid Overlays over Cement Concrete Pavements:
Empirical formulae have been evolved by certain organisations such as the U.S. Corps of Engineers for the design of rigid overlays over cement concrete pavements.
The Indian Roads Congress have given their recommendations for the design of overlays on cement concrete pavements in a special publication: “IRC: SP: 17-1977: ‘Recommendations about overlays on cement concrete pavements,’ Indian Roads Congress, New Delhi, 1977.”
These recommendations have been based on extensive field studies on the performance of overlay works in different states of the country.
Salient features of this special publication are presented herein:
Condition of the Existing Pavement:
The condition of the pavement may be categorised based on the total length of crack per unit area.
The following criteria are for guidance:
Rigid overlays as well as flexible overlays are possible. In the case of rigid overlays, fully bonded or monolithic, partially bonded, and unbounded types are based on the condition of the existing pavement.
In the case of flexible overlays, the overlay can be fully bituminous, or partly bituminous and partly granular. The choice appropriate to a given situation should be made after studying the relative economic considerations.
(b) Rigid Overlays:
Fully bonded and partially bonded overlays may be used if the existing pavement is sound or slightly cracked. Partially bonded overlays may be used if the existing pavement is ‘fairly’ to ‘moderately’ cracked. Un-bonded overlay with a 50 mm bituminous separation layer may be adopted if the slab is ‘moderately’ or ‘badly’ cracked.
(i) Partially Bonded Overlays:
The thickness of the overlay (h0) may be got from the following equation –
Where, hm = thickness of the monolithic slab
he = thickness of the existing concrete pavement
C = pavement condition factor
(ii) Un-Bonded Overlays:
The C-Values given above apply here also. For unbounded overlay, a separation layer is to be provided after the cracks or cleaned and sealed.
The thickness of the overlay is taken as the difference between the monolithic thickness of cement concrete pavement required for the present traffic and the existing thickness. The monolithic thickness has to be designed in accordance with “IRC: 58-2002: ‘Guidelines for the design of rigid pavements for highways,’ IRC, New Delhi-2002.” Proper bond must be ensured between the overlay and the existing pavement by thorough cleaning of the latter.
For very heavy traffic (> 1500 cvd), steel reinforcements at the rate of 3 kg/m3 should be provided in fully bonded and partially bonded overlays. Joints in these should be matched with those in the existing pavements both in regard to location and type.
(c) Flexible Overlay over Rigid Pavements:
A. The following specifications are recommended in areas of medium, intensity rainfall (> 40 cm and up to 125 cm), with favourable drainage conditions, and subgrade soil of low plasticity (PI not more than 14):
1. For Very Heavy Traffic (More than 1500 cvd):
(i) 7.5 cm bituminous macadam under 4 cm asphaltic concrete
(ii) 5 cm granular or layer (7.5 cm BUSG over 7.5 cm WBM) under 4 cm AC
2. For Heavy and Medium Heavy Traffic (151-1500 cvd):
(i) 7.5 cm BM under 2 cm premix with seal coat
(ii) 7.5 cm BUSG under 2 cm premix with seal coat (for medium heavy traffic only)
(iii) 15 cm granular layer under 2 cm premix with seal coat
(iv) 7.5 cm granular layer under 4 cm AC.
B. The following specifications may be adopted in areas of heavy rainfall (> 125 cm and upto 200 cm), unfavourable drainage and subgrade soil with high plasticity (PI 20 or above):
1. For very heavy traffic (more than 1500 cvd) – 10 cm BM under 4 cm AC
2. For heavy and medium heavy traffic (151-1500 cvd) – 7.5 cm BM under cm AC
C. The following specifications may be adopted for very heavy traffic (more 1500 cvd) in areas where the rainfall is very heavy (more than 200cm per year), drainage condition unfavourable, and subgrade of high plasticity:
(i) 5 cm coated macadam consisting of 40 mm single size aggregate mixed with 2.5 – 3% bitumen under 7.5 cm BM with 4 cm AC as wearing course
(ii) 11 cm BM with 4 cm AC as wearing course
The strengthened cement concrete pavement should be provided with adequate drainage facilities for successful performance.
The technique of providing cement concrete overlays on bituminous pavements is known as white-topping. This is expected to save significant quantities of cement.
The flexible pavement is taken to be a base course and its modulus of subgrade reaction is determined. The overlay thickness is determined using the design procedure for rigid pavements in the normal way. Kadiyali, et al, recommended that the value of K be restricted to 14 kg/cm3.
Before laying the concrete overlay, the profile of the existing pavement is made good by a levelling course of lean concrete or bituminous mix.