Pavement deisgn and analysis

Pavement design

(Thanks to professor Animesh das and Professor Partha Chakroborty.)

(http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/transport_e/TransportationII)

The objective of this lecture module is to introduce the basic concepts of analysis and design of pavement structure.

Also a discussion has been placed as past, present and future perspective of pavement analysis and design.

Types of pavement structures

• Pavements, in general, can be classified in two major categories: concrete pavement and bituminous pavement.
• Concrete pavements are generally called rigid pavements and bituminous pavements as flexible pavements.
• There could be some other types of pavements which are neither rigid, nor flexible, for example, block pavement, composite pavement.

A Pavement is a multi-layered structure. The layers are placed horizontal one over other. In general, the strengths of the layers decrease from top towards bottom except some special situation like inverted pavement. The terminologies used to identify various layers of bituminous and concrete pavements are identified in Figs. 1and 2

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Fig. 1 cross section of a typical bituminous pavement (Chakroborty and Das 2003)

Fig. 2 cross section of a typical rigid pavement (Chakroborty and Das 2003)

Bituminous pavement

• The subgrade is a compacted soil layer.
• The base and sub-base course could be made up of bound or unbound granular layer. As per Indian specifications (MORT&H 2001), some examples of base or sub-base layers are: Granular sub-base (GSB), Water Bound Macadam (WBM), Wet Mix Macadam (WMM) etc.
• The binder course is made up bituminous material. As per Indian specifications (MORT&H 2001), some examples of binder course are: Bituminous Macadam (BM), Dense Bituminous Macadam (DBM) etc.
• The wearing course is the top bituminous layer which is comes in contact to the vehicle tyre. Wearing course provides impermeability to the pavement surface against water percolation (Chakroborty and Das 2003). The binder course and wearing course together are called bituminous surfacing.

Concrete pavement

• Concrete pavement is, in general, consists of three layers, subgrade, base layer and the concrete slab.
• Generally bound base layers are used for concrete pavement construction. As per Indian specification, some example of such base layers are Dry Lean Concrete (DLC), Roller Compacted Concrete (RCC) (IRC:15-2002)
• The concrete slab is generally of M40 to M50 grade of concrete as per Indian specifications, and is called as paving quality concrete (PQC) (IRC:15-2002).

Joints in concrete pavement

Fig.3 Location of joints in concrete pavement

• Joints are the discontinuities in the concrete pavement slab, and help to release stresses due to temperature variation, subgrade moisture variation, shrinkage of concrete etc.
• There are various types of joints in concrete pavement, e.g. contraction joint, construction joint, expansion joint and warping joint. Fig. 3 schematically shows position of various joints. The functions of these joints are as follows:
• Contraction joint: Contraction joints are provided along the transverse direction to take care of the contraction of concrete slab due to its natural shrinkage.
• Construction joint: Construction joints are provided whenever the construction work stops temporarily. The joint direction could be either along the transverse or longitudinal direction.
• Expansion joint: Expansion joints are provided along the transverse direction to allow movement (expansion/ contraction) of the concrete slab due to temperature and subgrade moisture variation.
• Warping joint: Warping joints are provided along the longitudinal direction to prevent warping of the concrete slab due to temperature and subgrade moisture variation.

These discontinuities (joints) could be extended to the full or partial depth of the slab. Sometimes iron bars are provided across the joints, the iron bars along the longitudinal joints are called tie bars and along the transverse joints are called dowel bars.

Pavement analysis and design: historical perspective

The past pavement design approaches were mostly empirical in nature and were based on experience.

• CBR method of pavement design is one of the earlier pavement design approach developed during 1928-29 (Ullidtz 1986). In this method the thickness design charts are developed (based on observation of number of sections), with reference to the sub grade CBR value for the most critical moisture condition. In 1940 this method was adopted by the U.S. Corps of Engineers for design of airfield pavements (Horonjeff and Mckelvey 1983). It is interesting to note that the design initially did not involve traffic as a parameter, which was introduced later as a correction factor. The method was further improved by considering the CBR values of the individual layers and thereby individual layer thicknesses are obtained. In some other approach, Hveem resistance value of pavement materials is used instead of CBR value.
• Another pavement design approach considers aspect of bearing capacity of the individual layers, and the design is finalized in such a way that the bearing stress does not exceed the bearing capacity of the individual layers. This method was first proposed by Barbar in 1946, and is still in use (TRH4 1996, deBruin et al. 2002), however this method does not seem to account for traffic repetitions.
• Another approach recommends limiting recoverable deflection as the criterion for pavement design (Huang 1993). Failure theories suggest that the failure of a structure is due to excess stress or strain, thus, deflection may not be attributed as basic pavement design criteria.

Pavement analysis and design : current perspective

Present practice of pavement design involves considerations of three aspects: structural design, functional design and drainage design and they are explained briefly in the following:

Structural design

In structural design the stresses due traffic loading and temperature are estimated, and the thickness of the pavement is designed in such a way that these developed stresses/ strains are below the allowable values. The current practice of pavement design, more popularly, is known as Mechanistic-Empirical pavement design and is followed by a number of organizations around the world (Asphalt Institute 1999, Shell 1978, Austroads 1992, NCHRP 2005, IRC 2001). It is mechanistic pavement design because it uses stress/ strain of a pavement structure using mechanics based principle, and, as well, it is empirical because the expected life for a given stress/ strain level is estimated from empirical relationships obtained from laboratory or field performance studies. The pavement design approach is not governed by the maximum amount of load that the pavement can sustain, rather, it estimates the number of standard load repetitions that can cause failure.

Estimation of pavement stress/strain

Stress/ strain due to load

• For pavement design purpose, the stress/ strain value of a pavement structure is obtained from structural analysis of the pavement (Ioannides et al. 1998). The stress/ strain values at any point of a pavement structure can be estimated when the elastic moduli, Poisson's ratio and the thicknesses of the individual layers are known. The strain values can also measured using strain gauges.
• Any analysis procedure involves idealization regarding the structure; similarly, measuring strain involves measurement errors - hence the true value of stress/ strain is never known.
• A concrete pavement slab, in general, has finite dimensions, and thus the analysis approach of concrete pavement becomes different than the analysis of bituminous pavement. For bituminous pavement, in general, the pavement is assumed as infinite in both the directions, whereas for concrete pavement, in general, it is analysed as discrete slabs connected by joints. The concrete pavement is also assumed to have bending moment carrying capacity, whereas flexible pavement is assumed to have no moment carrying capacity.

Stress/ strain due to temperature

• The change of temperature causes the pavement to expand or contract. The restriction of free movement causes temperature stresses.
• There exists temperature variation across the depth of the pavement - this causes warping stresses.
• The temperature stress varies across the corner, interior and edge of the concrete slab, also at different times of the day. The most critical combination of load and temperature stress is used as design criteria.
• The temperature stress in bituminous pavement is insignificant. Hence, temperatire stress, is not considered in pavement design. However, temperature affects the elastic modulus of the bituminous layer, which needs to be duly considered in pavement design.

Estimation of layer thicknesses

• The thickness of the pavement is adjusted in such a way that the stress/ strain developed is less than the allowable values obtained from past performance information.
• The two major modes of structural failure of pavement are fatigue and rutting.
• Fatigue: Traffic applies repetitive load to the pavement surface, and the cracks start from bottom the bound layer/ slab and propagate upwards. When the extent of surface cracks reaches a predefined level, the pavement is said to have failed due to flexural fatigue.

· Conventionally, for design of concrete pavement stress is used as parameter, and for design of bituminous pavement strain is used as parameter.

• Rutting: Rutting is the accumulation of permanent deformation. This is the manifestation of gradual densification of pavement
• The vertical strains on the pavement layers, mainly the vertical strain on the subgrade is assumed to be governing factor for rutting failure.
• The rutting issue is not considered for concrete pavement design, because it does not have any permanent deformation.
• The fatigue/ rutting equations are developed from field or laboratory studies, where fatigue / rutting lives are obtained with respect to respective stress/ strain for fatigue/ rutting. For a given design life, thus, allowable fatigue and rutting stress/ strains can be estimated using the fatigue/ rutting equations.
• The various other types of pavement failures could be shrinkage, thermal fatigue, top down cracking (for bituminous pavement) etc.

• layers, and shear displacement of the subgrade.

Design of joints

The spacing of the contraction joint is estimated from the shrinkage potential of concrete. The spacing of the expansion joint is estimated from the coefficient of thermal expansion of concrete, maximum change of temperature and the acceptable joint gap. Since, the concrete is good in compression, the experience over last few decades indicates that concrete pavement can be constructed without any provision of expansion joint (ACPA 1992). The dowel bars are designed by assuming that they participate in the load transfer, when the vehicle moves from one slab to other. The tie bars are designed in such way that they have enough strength to tie the two adjacent slabs. The design of dowel bar and tie bar is discussed in detail later.

Functional design

The functional pavement design involves considerations of skid resistance, roughness, surface distresses, reflectivity of pavement surface etc. The functional pavement design considers mainly the surface features of a pavement.

Drainage design

A road needs to be designed in such a way that the rain/ snow precipitation is drained off the pavement and its surroundings. A suitable surface drainage system for the pavement is designed for this purpose. Some water, however, will percolate into the pavement from its top surface and needs to be taken out of the pavement - this is done by providing an internal drainage system to the pavement. Water will also try to enter into the pavement from bottom due to capillary rise or due to rise in water table. A suitably designed sub-surface drainage system tries to avoid such a problem.

Pavement analysis and design: future perspective

• Mix design, quality control, construction method and pavement design together determines the performance and longevity of a pavement. The future pavement design is expected to take an integrated design approach considering all these issues together.
• The parameters associated with pavement design are stochastic in nature. Thus, the two pavement designs (designed deterministically) having same design traffic may have different levels of reliabilities of survival. Thus, reliability issues of pavement design are gradually becoming important considerations (NCHRP 2005).
• A pavement designer essentially looks for the most economical design, yet considering the structural, functional, and drainage design requirement. The future pavement design practice is expected to consider the cost optimality over the entire life cycle of the pavement (Abaza and Abu-Eisheh 2003).

The present module highlights the basics of analysis and design of bituminous and concrete pavements. Pavements are analysed as layered horizontal structure with given elastic module and Poisson's ratio. Concrete pavement, in general, is made up of discrete slabs - therefore, it has joints both in longitudinal and transverse direction. A pavement is designed from structural, functional and drainage considerations. Fatigue and rutting are two major modes of structural failure of pavements. For concrete pavement design, temperature stresses are also considered along with stresses due to load. A pavement designer does not design a pavement for the ultimate load the pavement can carry, rather, the number of repetitions that the pavement can sustain