Various design approaches

A brief discussion on concrete pavement design approaches suggested by various design practices, viz. PCA method (1984), Austroads method (2004), AASHTO method (1993), NCHRP mechanistic-empirical method (2004) and the Indian Roads Congress (IRC) method (2002) are placed in the following:

Portland Cement Association (PCA) Method

The PCA method is based on Westergaard, Picket and Ray's work and further theoretical analysis by Finite Element Method (Huang 1993). The data used to develop the PCA method is generated from various road tests, like, ASSHO road test, Arlington test (conducted by PCA), Bates test road, and Maryland road test (PCA 1984). The PCA design method is based on the following two considerations (PCA 1984):

• The fatigue damage on the concrete slab, due to repetitive application of traffic load is estimated. The cumulative fatigue damage principle is used to estimate the design thickness of the slab. Edge stress between the mid-way of the transverse joint is taken as critical configuration.
• The stresses due to warping and curling (due to temperature and moisture gradient) are not considered in the fatigue analysis as per PCA recommendation, because, most of the time the stresses generated are subtractive to the load stress.
• The possibility of erosion of pavement materials placed below the concrete slab is evaluated. The rate at which the slab is deflected due to axle load is used as a criterion for erosion. Theoretically, it can be shown that a thin pavement with smaller deflection basin is subjected to faster rate of deflection, compared to thicker slab. Hence thin slab is more susceptible to erosion. In a similar way the cumulative erosion damage is calculated for individual axle load groups. If this value is greater than one, then the design needs to be revised.

Austroads method

Austroads method has been adopted from PCA (1984) approach with modifications suited to Australian conditions (Austroads 2004). A bound mix or lean cement concrete is used as sub-base material. For a given CBR value of the subgrade and given thickness of cemented sub-base, the effective subgrade strength can be obtained from the chart provided. Figure-19 schematically shows such a chart. For different levels of traffic, Austroads (2004) suggests minimum values of the base thicknesses to be provided.

Figure-19   Schematic diagram of Austroads (2004) chart for estimation of effective subgrade strength

As per Astroads (2004), the concrete pavement slab thickness, for a given expected traffic repetitions, is designed considering the (i) flexural fatigue of the concrete slab and the (ii) subgrade erosion arising out of repeated deflections. For both the considerations, equations are suggested to calculate the allowable traffic repetitions. If the allowable traffic is less than the expected traffic, the design is revised by increasing the slab thickness. The concrete shoulders adopted are of integral type or structural type .

AASHTO method

The AASHTO method (1993) for design of concrete pavement has evolved from the AASHO road test (AASHO 1962). The AASHTO pavement design follows an empirical approach. Pavement performance in terms of present serviceability index ( PSI ), loss of serviceability, subgrade and sub-base strength, cumulative traffic, properties of concrete, joint load transfer efficiency, drainage condition, overall standard deviation and reliability are the input parameters considered in the pavement design.
The PSI value of the fresh pavement is assumed as 4.5 and the pavement is deemed to have failed when the PSI value reaches 2.5. The resilient moduli of the subgrade and sub-base materials are determined in the laboratory simulating the seasonal moisture content and stress situation. Suggested values are also available for given moisture content, plasticity index etc. The composite modulus of subgrade reaction (k) is estimated from modulus of subgrade reaction and elastic modulus of sub-base for various seasons and the depth of rigid foundation and the thickness of the sub-base.
Design equation as well nomographs are available to estimate the slab thickness (D) from these input parameters.

NCHRP mechanistic-empirical (M-E) method

The NCHRP (2004) has recently developed concrete pavement design procedure based on mechanistic-empirical (M-E) approach. This approach attempts to reduce the extent of empiricism prevalent in the existing AASHTO (1993) guidelines. This proposed NCHRP pavement design system is modular in nature, that is, the design approach can be modified by parts (as and when new knowledge is available) without disrupting the overall design procedure. This approach also can take care of various wheel-axle load configuration (Khanum et al. 2004).
As per this approach trial thickness of the slab is first assumed, and the stress, strain and displacement values are obtained. From these values, the performance of the pavement in terms of distresses (such as faulting, cracking) and smoothness and predicted. If these predicted performance parameters does not satisfy the required performance for a given reliability, the design revised.
The design approach includes a large data-base as input parameters, for example, average daily traffic, traffic growth rate, traffic composition, hourly weather data on air temperature, precipitation, wind speed, percentage sunshine, relative humidity, pavement material engineering parameters, ground water depth, infiltration, drainage, hydraulic conductivity, thermal conductivity, heat capacity etc. The temperature stress is considered in this method, but the temperature profile is linearized to enhance computational efficiency (NCHRP 2004).

Indian Roads Congress (IRC) method

The Indian Roads Congress (IRC) guidelines, IRC:58 (2002), has adopted the Westergaard's equation to estimate load stress and Brdabury's equation to estimate temperature stress. The load stress is highest at the corner of the slab, lesser in edge and least in the interior. The order of variation of temperature stress is just the reverse of this. As per IRC:58 (2002), it is recommended that the design needs to be done for edge stress condition and subsequently check needs to be performed for corner stress condition so as to finalize the design. The following are the steps followed as per IRC:58 (2002) guideline for the design of concrete pavement:

• The input parameters are obtained to formulate the design problem. The joint spacing and the slab dimensions are decided. If there is a bound sub-base layer over the subgrade, a suitable value of effective k is to be adopted.
• A trial thickness of the concrete slab is assumed.
• The edge stress is estimated for various axle loads from the given charts. Figure-20 schematically shows such a chart. The cumulative fatigue damage principle for fatigue is applied to check the adequacy of the slab thickness.
• The sum of edge stress due to load for the highest axle load group and the temperature stress should be less than the MOR of concrete, otherwise the design is revised. 
• The  adequacy of corner stress is checked with respect to MOR value and accordingly the design is finalized. Westergaard's corner stress formula is for estimation of corner stress due to load, and the corner stress due to temperature is assumed to be zero.
  

Figure-20 Schematic diagram of IRC:58 chart for estimation of load stress at the edge (IRC:58 2002)