Concrete pavement design methods

Some of the methods for design of concrete airfield pavement structures are (i) Corps of Engineers (CoE) method (ii) Federal Aviation Administration method (FAA) and (iii) Portland Cement Association (PCA) method and are discussed briefly in the following:

US Army Corps of Engineers (CoE) method

The CoE method uses fatigue equation developed from full scale load tests conducted during 60's and 70's (Smith et al. 2002). The current Unified Facilities Criteria (UFC 2002) considers fatigue of pavement slab due to edge stress. The edge stress is calculated as per Westergaard's analysis and assumes that load transfer joints reduces the stress by 25% ( Thuma and Lafrenz 2003 ). The method allows reduction of slab thickness if the subgrade strength is high.

Federal Aviation Administration (FAA) method

The standard design curve assumes aircraft wheels are either placed parallel to or perpendicular to pavement joint. Westergaard analysis of edge stress calculation is employed. It is assumed that 25% of the edge stress is carried by the joints at the edges (FAA 2006). The following are the considerations for design of concrete pavement as per FAA (2006):

• The modulus of subgrade reaction is estimated by performing plate load test on the subgrade. If the subgrade is multi-layered, or sub-base (granular or stabilized) is put over the subgrade, the effective modulus of subgrade reaction needs to be determined. Charts are provided, similar to the Figure 19, presented in the lecture on 'various design approaches'.
• Same concept, as discussed above for the design of bituminous pavements, is applied to find out the design aircraft .
• Design charts are available, which involves the input parameters as, concrete flexural strength, effective modulus of subgrade reaction, gross weight of the design aircraft and the annual departure of the design aircraft. The cumulative fatigue damage principle is employed to arrive at the design thickness of the slab. A typical concrete pavement design chart as per FAA (2006) is schematically presented in Figure 26. The dashed line indicates the order of progression.
• Similar strategies are adopted, as discussed for design of bituminous pavement by FAA method, to take care of the frost penetration problem.
• Standard schemes are recommended (FAA 2006) for providing contraction, construction and expansion joints to the pavement. Provisions are also provided for jointed reinforced concrete pavement (JRCP) and continuously reinforced concrete pavement (CRCP).

  A worksheet software for design of concrete airfield pavement as per FAA can be downloaded from the FAA website:

Figure 26: A schematic concrete pavement design chart as per FAA (2006)

Portland Cement Association (PCA) method

The fatigue equation of PCA method is developed from laboratory testing of fatigue behaviour of concrete beams (Smith et al. 2002, Packard 1995). The PCA method (Packard 1995) requires the (i) concrete properties (ii) effective strength of subgrade (or subgrade-sub-base combination) (iii) type of aircraft, load, and approximate frequency and (iv) type of pavement being designed, such as runway, taxiway, apron hangar etc. Use of 90 days strength test results of concrete is generally recommended for design purpose. The design procedure can be summarized as follows:

• The modulus of subgrade reaction is estimated by performing plate load test on the subgrade. If the subgrade is multi-layered, or sub-base (granular or stabilized) is put over the subgrade, the effective modulus of subgrade reaction needs to be determined. Charts are provided, similar to the Figure 19, presented in the lecture on 'various design approaches'.
• From careful estimate of the present and future traffic, an appropriate safety factor is estimated for each of the aircraft types. The working stress is obtained by dividing the modulus of rupture of concrete with the safety factor chosen for individual aircrafts. Suggested values of safety factor range from 1.7 to 2.0 for critical areas and 1.4 to 1.7 for non-critical areas.
• From the pavement design chart, the design thickness slab thickness is estimated for a particular type of aircraft. Figure 27 presents a typical pavement design chart as PCA (1995). The cumulative fatigue damage principle is employed to arrive at the design thickness of the slab. The dashed line indicates the order of progression. This process is repeated for various aircrafts and the highest value of the thickness is chosen as the design thickness.
• The stresses created due to less critical aircrafts are also checked (by using the design charts in the reverse direction), and if the values are less than some specified value (350 psi), it is assumed that these do not add to the fatigue of the pavement slab.

  
  Figure 27: A schematic concrete pavement design chart as per PCA (1995)